Did you remove all of your cookies from scubaboard and try again?
This was a problem with vBulletin 3... But has not been reported with vBulletin 4. So I would have to believe it's related to the machine you're on. Either it's not storing cookies at all, or you have too many cookies and it's confused.
Can I report a problem with it now? I just started using Chrome (SB seems to load a lot faster than IE9), tried logging in for the second or third time ever via Chrome and the login redirect brings me back to the unlogged in Home page. If I click the Forum tab, it'll show me as logged in. As a test, I deleted all my cookies (all 14 of them, or something) and reopened SB. same thing. I closed SB and reopened it to the Home page, not logged in though I should have been cause I had "remember me" checked. I click the Forum tab and bingo, I'm logged in, so cookies not getting saved isn't the issue. It looks like if I get to the Home page from anywhere other than a page on SB that shows "Welcome, oreocookie" at the top, it'll show me the guest version of the page.