I had this same problem and, after weeks of frustration, just recently found a solution (no thanks to Apple support):
First off, perform a software update and reboot your Mac, which assures the .me updates have been fully implemented. I'm using OSX 10.5.4 and iWeb 08' 2.0.4. Then, in iWeb, perform a 'Publish All to MobileMe' from the 'File' pulldown.
Early visitors of my website (prior to '.me') had bookmarked my site and (logically) were using the bookmark to return there. Well, their bookmark url pointed to 'web.mac.com/.....' and this url allows visitors to 'view' your website, including your BLOG page.....but when attempting to 'Add a Comment', the error message you stated shows up. I was under the impression that Apples .Mac to .Me transition was suppose to be seamless (meaning, no user input was needed) so the url didn't raise any suspicions. While on a iWeb BLOG page, look at the url in the address bar of your web browser....the url must read 'web.me.com/.....'. If it doesn't, edit it and press return.....now try to 'Add a Comment' on your BLOG page. It should work.....it did for me anyways. Make sure to re-bookmark with the correct url (web.me.com/......) and notify the early visitors of your website to the solution.
After I did a software update, rebooted and performed a 'Publish All to MobileMe', new visitors to my website do not have issues adding comments to my BLOG page.
The short of it....when you're on the BLOG page, to be able to "Add a Comment', the url must read 'web.me.com/......'.
For me, the combination of steps I outlined above solved the problem of not being able to 'Add a Comment' on my BLOG page. Hopefully, I have written this in a manner that is understandable.
Hope this helps.
to check out mine....click
http://web.me.com/srhaberer/My_Journey/Welcome.html
PS - On my Welcome and Blog page, I added a note for visitors to click this 'link' and re-bookmark.....when I used the iWeb feature to create the 'link', I selected 'one of my pages'.....the url iWeb used was 'web.mac.com' (which only continues the problem).....I had to use 'an external page' reference. Can you believe this? What are Apples programmers thinking? Someone is sleeping at the wheel, IMO.