Good news, people: Gmail now supports rich text in its signatures. And that's not the only important update to Gmail signatures this week.
First, the best news: Go to Settings and you'll see the ability to create rich text signatures. You can fiddle with font, size, style, and more via the toolbar. You can also add images (though only if they're stored online somewhere) and embed links. Right out of the gate, for example, I created a rich signature that linked to my card.ly vCard. (My only complaint? There's no apparent way to add a link to an image.)
And, as I mentioned earlier, that's not all. Gmail now lets you create a custom signature for each e-mail address you have Gmail configured to send from. So, if you've set up a different account on the Accounts and Import tab (click Send mail from another address), you can automatically send the right signature for whatever e-mail account you send from.