IOS – Prevent Gmail signatures from syncing between iPhone and iPad

Check to see if Gmail on your Mac is set up to use IMAP instead of POP3 by going to Preferences > Accounts in Mail. IMAP syncs the read / unread status (among a host of other benefits).

If it's not set up to use IMAP then you'll have to remove the account and add it again, this time making sure to select IMAP in the account wizard.

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.