Jamey,
There is a way round this. First of all change the setting that automatically puts a signature in an e-mail.
Tools > Options > Mail format tab
Then change Signature for new messages and signature for replies and forwards to <None>
This means that when you click New message there won't be a signature. Not to worry that will be sorted doing this.
Create a new message, choose which Account/Address to send from, to add the signature either do:
Insert > Signature > My signature (Whatever you've called it) or by click on the Signature button that looks like a piece of paper with a hand holding a pen and select the right signature.
This solves the problem of having a signature only for one address but it does involve a click on the mouse.
Hope this helps in some shape or form.
