Up to now (with iOS < 6.0), I used the following URL and code to send the user directly to the review page of my app (to rate it with 5 stars and post a friendly comment :))
I guess apple almost fixed the issue, since it can go to the review page directly with the same address again. Users still need to tap the button of review on iOS6 after they are directed to the review page though.
Use url like this
https://userpub.itunes.apple.com/WebObjects/MZUserPublishing.woa/wa/addUserReview?type=Purple+Software&id=561892747&mt=8&o=i
I found it on a link to "write a review" in Apple receipt email.