Add a second signature in iPhone/iPad Mail
iOS only supports a single signature, but you can use iOS 5's new text Shortcut feature to manage multiple signatures. The feature is in the Settings app under "General | Keyboard | Shortcuts".

Add a shortcut for each signature, e.g. wsig = Work Signature and psig = Personal Signature

Shortcut only supports single lines of text, so I get around this by having my default sig be my name plus a new line, then I type the shortcut text on the blank line after my name.