We've noticed an issue on machines we've updated to Windows 7 SP1 accessing mapped drives as an administrator. UAC is enabled, and it appears to be specific to a process or command prompt running as administrator.
Say for a drive M:, mapped to \\foo\\bar
If we run a standard command prompt and type "dir M:" normally (non-admin), it works fine. If it's from an admin command prompt, it now receives "The system cannot find the path specified.". Yet from the same admin prompt, dir "\\foo\bar" works fine - it is only the mapped drive letter that fails.
Whether the share is read-only or read-write does not appear to matter. Further info has shown that it was not whether the account was an "owner" of the share (think homedrive) that mattered, but how/when it was mapped.
As for why the command prompt is running as admin - that is because it's a developer environment for compiling software and it needs admin in order to register DLLs. It's also common when debugging...
We've been on Windows 7 for a while. I'm surprised this worked at all, given the link @pk provided. The shares in question are mapped during login...so it working/not working may be tied to that.