Guys, I have an update: it is a confirmed bug in Vista, namely the driver tdx.sys that handles the TDI compatibility stack.
We have a pre-release fix from Microsoft, and it does solve the problem. However, since the fix is not yet official, we're not allowed to give it to anyone outside our organization.
Once the fix the officially released, I'll let you know (hopefully you won't be required to go thru the MS Tech support hell to obtain it).
BTW the bug will be documented in Microsoft Knowledge Base article 933049.
Thanks
Vlk