If I had the funds I think I'd audition the Magnepan "Mini-Maggie" 2.1 system.
At just a tick under $1500 for the speakers alone they have been called the "finest desktop speaker system" and listed as "Recommended 2011" by AV Guide. You'll need to add the cost of an amplifier or receiver to this.
Inner Fidelity called the Mini Maggies "as close to virtual reality as hi-fi ever gets" and "I felt like I was in the first row hearing Simon and Garfunkel directly".
Well regarded audio expert Steve Guttenberg (no, not the actor) is quoted as saying "Mini (Maggie) is, hands down, the best-sounding desktop speaker you can buy" in his December 2011 review.
Full size Magnepan planar ribbon speakers have always been highly reviewed and are a completely different technology than the typical cone based speaker. The advantages of planar speakers are a much larger surface area producing the sound with a lower mass which translates to a wider soundstage with a much faster response and accuracy.
Because of their design planar speakers are ultra thin and I think they would go perfect with the new "thin" iMac.
Check out the side view:
My main home audio system speakers are Martin Logan electrostatics which are yet another variety of planar speaker. As such, I've always wanted to try a set of Magnepans.
A less expensive option would be the much larger floor standing Magnepan MMGs which come it at just around $600. Obviously they are not as convenient for use as computer speakers though...
Right now, at my desk I'm using a pair of original Infinity Modulus speakers.
These are the ones with the Emit K ribbon tweeters (MSRP $3000), rated as a Stereophile Magazine Recommended Class B speaker, and sold at hifi boutiques. I have two sets of these, one purchased new and one used off of Audiogon. These are not to be confused with the later Modulus II 5.1 plastic home theater speakers (MSRP $1200).