How can I add some html to my Mail.app signature?
It's not a huge effort to go through, but it does involve a little effort on your part. I haven't tried this myself (though will try it shortly just out of interest), but the basic principle seems to be to make a .webarchive file and drag it into the signatures preference box.

Worked example
eHow article
MacWorld example

I went and tried this and it does work in the current version of Mail.app, on Snow Leopard. The trick is essentially;


Create a .webarchive of your desired signature.

Bear in mind that any graphics you put in there need to be graphics that are out on the 'open web', rather than references to files on your own computer etc. I think it's this bit that catches a lot of people out.
Go into Mail and go through the motions of creating a new signature.
In the Library/Mail/Signatures folder you'll see Mail creating a new file with a guid-like filename (eg 61F51203-5C18-4061-A8EF-F53B17C41476.webarchive). Make a note of this filename!
Close down Mail, and replace the contents of that signature file
with your webarchive file.

The best way of doing that is;


Paste your webarchive file into the same folder (eg rob.webarchive), then highlight the new file that Mail created and press return - the filename should become selected for you.
Press cmd-c to copy this filename, and then alter the filename (eg add 'old-' to the front of the name). Press Return to close the 'name editing' session.
Now select your own webarchive file (eg rob.webarchive), and again press return to edit the name. Now cmd-v in the name you copied from the first file, and press Return to close the editing.

When you restart Mail, the new signature you've made will be available.


Gotchas include;


you're linking to images, css files or whatever that are not available on the internet,
you're not composing your message in 'rich text' form.


But I've just been through this and even though I have no use whatsoever for a HTML signature, it does work. :-)