It is possible to create an HTML signature for Mail on an iOS device without jailbreaking that device.
For this to work you need a backup of your iOS device made via iTunes.
First, create your HTML signature in your favorite text editor. It is essential that you replace the HTML tag delimiters < and > with the appropriate HTML entities: < and >.
Next, run Then run iBackupBot, choose your latest backup, then look for /Library/Preferences/com.apple.mobilemail.plist. Click on this file to display it in the editor, and find the <SignatureKey> section.
Below it you will find
Exit iBackupBot, then restore your iOS device via iTunes. (Click on the device in the iTunes source list, then Option-click on the Restore button and select your backup.) The signature should then appear in any e-mails you send from that device.
[kirkmc adds: I haven't tested this.]
Mac OS X Hints