WARNING: Geek/programmer stuff ahead
The problem with the WotC boards is that they're running them on a forked vBulletin (PHP/MySQL running from Apache on Linux [Red Hat, I believe]), but all of Gleemax is run via ASP.NET on Windows (Server 2003, probably). This alone means they're either storing the session in the database, or doing some really hacked-up programming to keep it going between Windows and Linux (I haven't tried that, but I've tried to maintain session between Classic ASP and ASP.NET, and it just didn't work due to the methods used to retrieve the data). They're so caught up in trying to integrate everything and have SSO (Single Sign On) that they aren't taking the steps to make sure it will work properly first. If they knew they were doing to want SSO capability, they should have either A: Written the Gleemax stuff in PHP and run it all on Linux (probably the easier option, since Gleemax was a "greenfield" application) or B: Redid the forums in ASP.NET.
Trying to make a Linux-based messageboard integrate completely with an ASP.NET login and site is probably the worst idea to pick from (which, of course, makes it unsurprising at all that it's the option they chose). It can be done, certainly, but it requires a lot of forethought and a very good sysadmin who knows what he's doing. WotC clearly has neither of these. Also, I recall reading once that the way they've set up their MySQL database is a bit wonky (I think it had to do with clustering, or space issues, but I can't remember the exact details), which almost certainly isn't helping things.
On the plus side, if I got off my lazy butt and finished working on my "Gleemax Killer" gaming social network site, I could probably steal people away from the real thing because it never works.