IE7 adoption rate is somewhat slower than some were predicting (2.3% on Polish market in week 21-27 Nov, according to Gemius rankings), but when doing web development You shold test both IE6 and IE7. Microsoft has recently released image for Virtual PC with IE7. Here is link to detailed info how to install and setup whole thing, to have IE6 and IE7 on one machine.
Maybe after April 1st when image will expire IE7 adoption rate will be much higher :))
Correction
Well I made so many errors in such short entry… First, this image is with IE6 not IE7, and first link points to VPC itself. Disk image with IE6 is available here