It's enough to make me tear hair out ...
I've been frustrated with LAN shares/SAMBA for a while. My wife has Vista Home Premium and it won't remember LAN credentials no matter what, so I ended up writing a script that logs her on to the home server every time she boots up. Unfortunately that would not help in my case because I keep getting locked out. I don't know how to fix that with a script triggered by a client PC that is locked out of the server.
However, I may have found a solution to this specific problem. I think I understand at least the broad outline of what you were saying about SSO, and I have no general fixes, but I may not have to fix all of that in order to solve the immediate problem.
There are at least 2 ways of entering credentials into Win 7 Credentials Manager. One is the way I have always done it: entering credentials when Windows asks for them. The other is to enter them directly in Credentials Manager. I don't recall that as an option in XP. Anyway, when I enter them on request from Windows Explorer, they apparently get stored with persistence of 'Session' and they go away on reboot. When I enter them directly into Credentials Manager they get stored with persistence 'Enterprise' and so far they have persisted through a couple of reboots. Unfortunately, although it appears that Windows 7 has at least 3 persistence settings for stored credentials (Session, Enterprise, Local Machine) I have not found any explanation for what they mean or how to specify/change the setting for stored credentials. I'm tempted to characterize that as an inexcusably stupid design decision on the part of MS but since I don't know/understand a lot about security I'll try to refrain from insulting the developers.
I'd rather not make the 2 passwords (Win 7 and WHS) the same because I do have a long term goal of making the WHS system visible on the web. I need a strong password for that. OTOH, for my personal PC I need a PW that my wife can actually remember. The one I use is obscure enough it will prevent casual hacking but it isn't one I would use for anything accessible to a determined effort from cyberspace, and WHS specifically requires strong passwords for user accounts that have remote access enabled. I may be too simplistic about this, but my impression is that if I lose the PC a strong password won't provide all that much data security anyway if somebody has any skills at all. For real security on my laptop I'd need a rather strong password AND an encrypted disk drive, but encrypted disks on a home PC strike me as an invitation to problems. Or am I wrong about that?
Anyway, as you observed elsewhere, this business of sharing is too complex for most of us at a time when MS SHOULD have been making it easier. It's a bit too much to expect all of us to upgrade every computer we own to Win 7 just to be able to share files on the LAN, and so far as I know there is not yet a corresponding upgrade available for WHS. WHS with SP2 and Power Pack 3 apparently does not fully address all of the compatibility issues. Here's hoping that my semi-fix takes care of the problem.