While there's certainly no shortage of dynamic email signature tricks on MacOSXHints, unfortunately they all seem to have broken when Apple released Tiger. I submitted a bug back in December and was told it was a known issue and that my bug was a dupe. It's March now, still no fix.
So what's a Cocoa hacker to do, but code up his own fix and inject it into Mail.app. And so I offer my free (GPL'd) DynSigFix (DSF), a haxie-style unofficial patch to Mail.app which re-enables AppleScript access to change signature text.
To be clear: This is a hack. It patches your running copy of Mail.app in memory. It could reduce your shiny G5 tower to a smoldering molten pile of ooze for all I know. It comes with no warranty. If it breaks, you get to keep the pieces.
That said, it works pretty well on my two Mac's and hasn't crashed Mail yet. And I have FINALLY have my rotating Fortune quips back on the bottom of my outgoing emails!
This patch only works on PPC Tiger for the time being. It's written in Python, so in theory it might be cross platform. Unfortunately I'm not sure if the underlying patching mechanism works on Intel, and I haven't an Intel Mac to try it out on. Reports of success or failure would be appreciated.
Running DynSigFix couldn't be simpler: Launch Mail, launch DynSigFix. DSF will bounce in your Dock for a second, then disappear. There will be a message in the Console log reporting success. After that, any old AppleScript (such as this one) to modify signatures should work as it did in Panther.
If you run Mail at startup, I think it should be safe to make sure Mail is listed near the top of your Startup Items and DSF at the bottom. That should give mail enough time to wake up before DSF starts poking at it. You'll need to re-run DSF every time you quit and restart Mail. You also shouldn't run DSF more than once per Mail launch, as there's an unavoidable leak in the patching process. It's not a problem once, but if you run DSF in cron or something, you're probably asking for trouble.
I've released this under the GPL, so if anyone cares to improve it, feel free. I'd love hear about any improvements so I can integrate them into my source. I'll be adding a bit of technical information and links to the bug tracker shortly.

