Update: iOS 4.2 seems to have fixed this and links now open the App Store app directly rather than the review page.
This article Dissecting iTunes links explains everything.
Heading straight to a review form
It’s also possible to send someone
straight to the review form for an
app.
https://userpub.itunes.apple.com/WebObjects/MZUserPublishing.woa/wa/addUserReview?id=337064413&type=Purple+Software
Simply replace the ID with your app’s
ID to create a URL for your app.
On a desktop or laptop, this will take
you directly to the review form in
iTunes for the app or item. On an
iPhone or iPod touch, this will open
MobileSafari, then open iTunes or the
App Store app on the reviews page,
leaving the user only one tap away
from writing a review.
To find out an item’s ID, right (Mac &
PC) or control-click (Mac) on a link
to the item in iTunes and select Copy
Link. The result will look something
like this.
http://itunes.apple.com/au/album/dig-your-own-hole/id19605549
http://itunes.apple.com/au/app/consume-mobile-isp-packages/id337064413?mt=8
The ID is the number in the URL after
“id’.
It might be worth noting that this URL
was created using iTunes 8. I don't
think there’s a way to create review
links in iTunes 9, so maybe this URL
is being phased out. Use with caution.
Update: @mareMtl has provided an
alternative review link for apps.
Please note that it only works on
iPhone OS devices.
itms-apps://ax.itunes.apple.com/WebObjects/MZStore.woa/wa/viewContentsUserReviews?type=Purple+Software&id=337064413