I just bootcamped my new 27" late last night. Didn't go nearly as smooth as with the old 2008 imac with an optical drive, but seems to have worked. Here's (roughly) what I did...
1. Printed out the bootcamp instructions - very important!
2. I had a disk of Win7 Home & an external optical drive. I used Disk Utility to make a .cdr file of it on my desktop (many instructions online about how to do this) and then manually renamed it .iso. The file had a long, strange name.
3. reformatted a 4gb thumb drive to MS-DOS(FAT) via Disk Utility
4. opened Bootcamp Assistant. It gave 3 check box options. I tried to do all at once on a 4gb thumb, but it was rejected as too small. So I did each option separately:
5. first: made a bootable disk with the .iso & the reformatted 4gb thumb (it just fit...the .iso file was just under 4gb)
6. second: downloaded the windows support software. I did this on my desktop, then copied it to a second 4gb thumb that was also reformatted as MS-DOS(FAT). The download took FOREVER though it was only three-hundred-something mb.
7. third: installed Win7 & decided on a partition size, etc. This is where I needed the printed instructions as there were a few greek-to-me selections I had to make & without the instructions I wouldn't have known what to do. Easy to screw up here.
Lots of restarts later (by the computer, not me) and Win7 went from looking like minecraft to a beautiful functioning OS. YAY! Well, ok, maybe I'll reserve the beautiful adjective for OS X.
Wow. That was a long post. Sorry. Hope it helps.