Login problems with sub-domains & SingleCommunity setting

I have narrowed this problem down to a single string inside the user cookie, but I'm not sure why this is happening.

I have a single MT installation with a main blog with several sub-blogs like so:

www.domain.com
indiana.domain.com
florida.domain.com

I have set the SingleCommunity option in mt-config.cgi because I would like to have a single sign-on across all of these blogs. I have also set it so that all of the cookies come from .domain.com.

Let's say I comment on www.domain.com then I go to florida.domain.com and try to comment. The comment form says I'm logged in and gives me the personal greeting. I type my comment, click submit and THEN I get an error saying my session has expired. This happens every time - even if it has been only a minute since commenting on one of the other sites.

I have looked at the cookies produce from doing this and there is a variance in the SID in the cookie. Otherwise they are identical (except, of course, the time stamp).

What am I doing wrong? Users are frustrated that they are traveling between sites and having this error pop up frequently. If they can't comment, they aren't "signed in" and shouldn't have the personalized greeting.

Thanks!