MS09-001 resolves three vulnerabilities in the SMB protocol implementation, two of them leading straight to unauthenticated, remote code execution (read: total ownership of affected systems on a first-come-first-serve basis) and a mere denial of service condition.
Before you start thinking that these are all bad things that may happen in your future and hence ignorable, take a moment to appreciate the facts.
All versions of Windows up to and including 7 are vulnerable in their unpatched state, firewalled systems may be spared yet corporate PC’s rarely are firewalled from one another – which would give a potential worm plenty of room to spread – and that, in fact, there is a rumour around the block that there may already be exploit code in the wild for one or more of these vulnerabilities.
Patch now. Nobody would benefit from two Downadup-sized epidemics in one month – except virus writers.