In Absentia is correct, depending on the style of the rim, there is very little difference between the rims until you get into deep v's and such. You won't find those in 27" sizes.
And mihlbach is right about pedal strike. Happened to my buddy on his (lower left) a few times.
The main thing is the tire choices available for 700's. Not a whole lot out there for 27". Most people run their 27inchers because its easier to do that rather than getting 700's AND having to buy new brakes to fit the new wheel size. And if you have an older frame (mine is a 1970) then you have to buy brakes with a longer reach.
You can get 27" rims in flip/flop configurations as well:
http://cgi.ebay.com/NEW-27-ROAD-BIKE...QQcmdZViewItem
700's (on left) & 27"
