I tried to install VS SP1 from the ISO (not the Web Installer) on a machine and ended up with a non successful install with the following error: Generic Trust Failure.
 
The log file said:

Possible transient lock. WinVerifyTrust failed with error: 2148204800
[3/9/2011, 10:6:29]Possible transient lock. WinVerifyTrust failed with error: 2148204800
[3/9/2011, 10:6:30]C:\Dev\VSSP1\VS2010SP1\VC10sp1-KB983509-x86.msp - Signature verification for file VC10sp1-KB983509-x86.msp (C:\Dev\VSSP1\VS2010SP1\VC10sp1-KB983509-x86.msp) failed with error 0x800b0100 (No signature was present in the subject.)
[3/9/2011, 10:6:30] C:\Dev\VSSP1\VS2010SP1\VC10sp1-KB983509-x86.msp Signature could not be verified for VC10sp1-KB983509-x86.msp
[3/9/2011, 10:6:30]No FileHash provided. Cannot perform FileHash verification for VC10sp1-KB983509-x86.msp
[3/9/2011, 10:6:30]File VC10sp1-KB983509-x86.msp (C:\Dev\VSSP1\VS2010SP1\VC10sp1-KB983509-x86.msp), failed authentication. (Error = -2146762496). It is recommended that you delete this file and retry setup again.

Since I didn’t want to download the 1.5GB ISO a second time, I tried the Web installer and this time it worked like a charm.  Was the problem with a corrupt download or a file missing a signature I can’t say.