This will sound completely obvious, but try it if you haven't already...it has worked for me.
About once a week, I can't access the DVC Member Site. (I use a Vista machine and IE8.) Over time, I realized this happens when I leave IE open and I am logged into the DVC site. I finally found if I closed all instances of IE, and then re-opened the DVC site, I am able to get back in (although it always forces me to re-log in...even though I previously told it to "remember me").
Two final possibilities: delete all the cookies on your computer, then close your browser and restart the DVC site. Finally, a hard reboot may help.
If you have tried all these already, hopefully someone else has some good ideas. Good luck!