Zosh is a $3 app that allows you to sign attached documents on your iPhone. Basically, you forward the emailed document to Zosh from the iPhone's mail app, then you open the Zosh app to sign it (plus you can add a date and stuff).
Testing the app, the signing process actually work pretty well. Maybe the neatest component of the interface is that the signature line auto tracks your finger, meaning that if your signature goes larger than the iPhone's screen, it'll just pan over.
However, there's one MASSIVE catch. You can only sign PDFs at the moment, not text files like .docs. For me, that's a dealbreaker (ladies), and hopefully one that will be remedied (through simple auto-conversion or something) in later versions. [Zosh]