I had this problem a while ago logging into perlmonks.com. I switched to perlmonks.org and staying logged in has been no problem. Switching between .com and .org still logs me out.
I just checked and that's how I have always been loggin on (i.e. perlmonks.org). Just out of curiosity I tried perlmonks.com, www.perlmonks.org and .com
All exhibit the problem the exact same way... but thanks for the thought anyhow.