I too have this issue not sure what triggers the bug but happens sometimes when I open my lid from stand by but then again happens other time also. What I do know is that its bl~#dy annoying and that when it happens, the only way to get it back is with a restart. Not even Fn+F2 works!! this is a bad issue that needs sorting, I’ve tried both driver but issue remains. this isn’t the only issue I’ve had with vista though, contrary to the stories of vista running great on 6400's I’ve had load of issues to name but a few:

• reports from vista that wifi driver & graphics drivers are slowing down start up etc
• the gadgets taking ages to load at start up despite the side bar its self loading relatively quickly and also only have two pre-installed gadgets running
• a performance issues under advanced tools telling me performance can be improved by turning off aero, err what (considering i have the highest spec 6400 they ever offered including 2gb 667mhz mem)!!!
• Recycling bin image sometimes doesn’t up date after emptying.
• basically too many issues to list probably all down to drivers on dells side but as it seems they cant be bothered to pull their finger and do something it looks like ill have to return to good old xp, what a waist considering I went for ultimate as well!!!