HTML/rich text email signatures in iOS?
There's a JailBreak Tweak called Mail Enhancer that does exactly what you want. It's $4.99 in the BigBoss repository, and there's another one called SIGnify for $1.50 in the ModMyi repo. They both allow you to have signatures with HTML formatting, but the more expensive one has many more features that make the Mail.app much more usable.