I am compiling a list of leather cases available for the iPhone 4. Leather cases often prove to be very luxurious and nice to have on your phone in any situation. Below is a list of companies that manufacture leather cases for the iPhone 4 along with a link to their case collection and an overview of up to three (depending on how many leather cases the manufacturer produces) of the company's stand-out leather cases.
Beyza
http://www.beyzacases.com/cat-apple-iphone-4-cases.cfm
Retro Super Slim Strap Vertical Case
Slim design with a strap that lets you take your iPhone out quickly. Available in a variety of colors and styles.
$44
Uni / Stripe Lateral Case
Utilizes a magnetic closure system, and a belt clip can be optionally added. Available in many different colors and designs.
$62
Slimline "Stripes"
A slim and padded design that is optimal for keeping your iPhone in a pocket. Available in a wide variety of color combinations.
$46
Case-Mate
http://www.case-mate.com/iPhone-4-Cases/iPhone-4-Cases.asp
Signature Leather Case
Italian Napa leather and a form-fitting design. A screen protecor is also included.
$34.99
Horizontal Hipster
Form-fitting Italian leather hipster. A screen protector is also included.
$39.99
Firenze
Premium Napa leather pouch. Available in brown and tan or black and grey (no pictures are available yet for that version).
$49.99
ION-factory
http://www.ion-factory.com/eskimo_iphone4.html
Eskimo Pouch
Made out of Nuback cattle hides, and each case has unique characteristics. Available in a single color or dual colors.
$89.89
Marware
http://www.marware.com/products/iPhone-4
C.E.O. Premiere
Eco-Leather hip case with a slim belt clip built-in.
$29.99
C.E.O. Flip-Vue
A flip-style case that includes a slim built-in belt clip and a pocket on the flap.
$34.99
SoBe
A vertical "purse-style" case with multiple compartments and a removable shoulder strap. A horizontal version is also available.
$44.99
Piel Frama
http://www.pielframa.com/apple-iphone-4-imagnum-cases.htm
iMagnum
Handmade with high quality cowskin in Spain, and has a thermoplastic polymer structure. Includes a belt clip and is available in black, brown, red, green, and blue.
$59
Sena
http://senacases.com/apple/iphone-4-cases/
Laterale
Includes a built-in belt clip and a velvet lining. Available in many different colors and variations.
$39.99
Walletbook
Both a designer wallet and a case. Available in many different colors and variations.
$52
Ultraslim Pouch
Very thin and light pouch with a velvet lining. Available in many different colors and variations.
$29.99
SGP
http://www.sgpstore.com/advanced_search_result.php?keywords=iPhone+4+leather&x=0&y=0
Argos
Premium ultra-slim leather flip-case. Available in black, pink, red, and white.
$37.99
Illuzion Legend
Premium ultra-slim leather flip-case. (Only available in black)
$37.99
Skin Guard Leather
A leather skin that is adhered to the back of the iPhone 4.
$17.99
Tuch
http://www.tuchmade.com/product/tuch-for-iphone-4
Tuch
Made out of one piece of leather with a built-in pocket, and the bottom is laser-cut for speaker/mic and dock connector openings. Available undyed (pictured) or dyed black.
Beyza
http://www.beyzacases.com/cat-apple-iphone-4-cases.cfm

Retro Super Slim Strap Vertical Case
Slim design with a strap that lets you take your iPhone out quickly. Available in a variety of colors and styles.
$44

Uni / Stripe Lateral Case
Utilizes a magnetic closure system, and a belt clip can be optionally added. Available in many different colors and designs.
$62

Slimline "Stripes"
A slim and padded design that is optimal for keeping your iPhone in a pocket. Available in a wide variety of color combinations.
$46
Case-Mate
http://www.case-mate.com/iPhone-4-Cases/iPhone-4-Cases.asp
Signature Leather Case
Italian Napa leather and a form-fitting design. A screen protecor is also included.
$34.99
Horizontal Hipster
Form-fitting Italian leather hipster. A screen protector is also included.
$39.99
Firenze
Premium Napa leather pouch. Available in brown and tan or black and grey (no pictures are available yet for that version).
$49.99
ION-factory
http://www.ion-factory.com/eskimo_iphone4.html

Eskimo Pouch
Made out of Nuback cattle hides, and each case has unique characteristics. Available in a single color or dual colors.
$89.89
Marware
http://www.marware.com/products/iPhone-4
C.E.O. Premiere
Eco-Leather hip case with a slim belt clip built-in.
$29.99
C.E.O. Flip-Vue
A flip-style case that includes a slim built-in belt clip and a pocket on the flap.
$34.99
SoBe
A vertical "purse-style" case with multiple compartments and a removable shoulder strap. A horizontal version is also available.
$44.99
Piel Frama
http://www.pielframa.com/apple-iphone-4-imagnum-cases.htm

iMagnum
Handmade with high quality cowskin in Spain, and has a thermoplastic polymer structure. Includes a belt clip and is available in black, brown, red, green, and blue.
$59
Sena
http://senacases.com/apple/iphone-4-cases/

Laterale
Includes a built-in belt clip and a velvet lining. Available in many different colors and variations.
$39.99

Walletbook
Both a designer wallet and a case. Available in many different colors and variations.
$52

Ultraslim Pouch
Very thin and light pouch with a velvet lining. Available in many different colors and variations.
$29.99
SGP
http://www.sgpstore.com/advanced_search_result.php?keywords=iPhone+4+leather&x=0&y=0

Argos
Premium ultra-slim leather flip-case. Available in black, pink, red, and white.
$37.99

Illuzion Legend
Premium ultra-slim leather flip-case. (Only available in black)
$37.99

Skin Guard Leather
A leather skin that is adhered to the back of the iPhone 4.
$17.99
Tuch
http://www.tuchmade.com/product/tuch-for-iphone-4

Tuch
Made out of one piece of leather with a built-in pocket, and the bottom is laser-cut for speaker/mic and dock connector openings. Available undyed (pictured) or dyed black.