I've just switched from a 3G to a 4, and immediately noticed the change in behaviour - portrait photos emailed directly from the 4's main camera are received as landscape. This was not the case with the 3G, and here's why:
Photographs taken with the iPhone4 main camera do not include thumbnail orientation tags
Dump the Exif info from any picture taken on a 3G with IOS3, and you'll see
Camera
Model : iPhone 3G
Orientation (start point): Right Upper
X Resolution : 72
Thumbnail
Thumb Compression : JPEG Compression
Thumb Orientation : Right Upper
Thumb X Resolution : 72
While on a 4, you get
Camera
Make : Apple
Model : iPhone
Orientation (start point): Upper Left
Orientation (start point): Upper Left
X Resolution : 72
Thumbnail
Thumb Compression : JPEG Compression
Thumb X Resolution : 72
Note that in the 3G image, there are separate orientation tags for thumbnail and main image, while the 4 (listed as just iPhone) incorrectly omits the thumbnail orientation, but includes two image orientation tags.
iPhoto is *incorrectly* interpreting the double orientation (i.e. displaying the wrongly tagged image incorrectly), whereas Finder's Quicklook (and every other image application out there) is *correctly* displaying the wrongly tagged image.
I suspect the bug crept into iOS4, and has gone unnoticed because it doesn't manifest itself in iPhoto.