iOS 5 still officially lacks support for multiple email signatures. If you've got more than one email account on your iPhone or iPad, then Lifehacker reader Ned Smith has provided a dynamite tip for creating multiple email sigs
Tap on Settings and then go to General | Keyboard. You can use the "Add New Shortcut..." option to create a series of keyboard shortcuts for each different signature. It works similarly to the autocorrect and autotext features in Microsoft Word. Ned's advice is to:
Create a shortcut for each line of your signature, and then when you need to add your signature to the bottom of an email you just punch in each shortcut followed by the return key. I use an abbreviation of the company name followed by an 'a' for line one, then 'b' for line two etc... (using a, b, and c etc... instead of 1, 2, 3 etc... is quicker because you do not have to switch to the numbers keyboard.)
So, perhaps set-up the default email signature (in Mail, Contacts, Calendars under Settings) with the generic information such as your name and mobile number. Then use the shortcuts for parts that change with each email account such as email address, landline and street address. Thanks Ned!