I installed SP1 for Windows Vista last night and I did not have any issues after the installation. The entire process took about an hour....just like the instructions from Microsoft advised. I did have to upgrade my sound card driver because Windows Update did not show SP1 available.
Steps I took to upgrade to SP1:
1) Downloaded stand-alone Vista SP1.
2) Upgraded sound card driver from Dell Support.
3) Installed Vista SP1
This is a great forum for Windows Vista, click here: Vistababble.com