I don't know why, but SQL Server 2005 Integration Services (SSIS) can't run a read-only package from inside BIDS. It doesn't seem to modify the file, since VSS Compare between the before and after versions says the file hasn't changed... Although the stack trace would lead one to believe that Visual Studio Integration is trying outsmart the user (developer) and just force a File | Save just in case the user (developer) forgot to... The file modified timestamp certainly changes.
So, self, the next time you get this fun, not-quite-helpful Annoying Message™:
Access Denied. (Exception from HRESULT: 0x80030005 (STG_E_ACCESSDENIED))
What it really means is that you've forgotten, again, to check out the package first, before executing it from Solution Explorer. Doh!