Important notice for iPhone 4.0 or later and iPad
Unlike previous versions, it is not possible to directly insert a signature into an email that you are replying or forwarding in iPhone 4.0 (IOS4) or iPad. Apple has changed the Mail app so that all inserts will always launch the a new email in compose mode. This is not a bug in our software, just some functionality that is not supported in the Mail app anymore.

To insert a signature into an email you are replying or forwarding, paste the signature from your clipboard. Each time you insert the signature through the Signature app, it will also add the signature to your clipboard. Subsequently, you can just paste the signature from the clipboard. (This also works for new emails too). There is no need to start the signature app each time as long as you don't overwrite the clipboard contents or you don't need to switch signatures.

Note: we only copy one signature at a time to the clipboard, the Mail app does not know how to choose from multiple signatures if we copied more than one to the clipboard.

How to paste from the clipboard:
Simply tap a blank area in your email where you want to paste your signature. A bubble with the word 'Paste' should appear. Tap the paste bubble to insert your signature or tap anywhere else to dismiss the Paste bubble.

If you find you had something else on the clipboard and that wasn't what you wanted to paste. Shake your iPhone and a dialog will appear to let you undo your paste.

Reminder you must have inserted a signature via the Signature app before to ensure the signature is copied to the clipboard. Once the signature is on the clipboard, you can use it as many times as you want by pasting from the clipboard without going back to the signature app. (unless of course you have copied something else e.g. from Safari to the clipboard).