There is an article on this point in the Watchguard knowledgebase: http://customers.watchguard.com/articles/Article/2166?retURL=%2Fapex%2Fknowledge Search&popup=false
Although this is directed at the iPod touch, I can tell you from personal experience it works for the iPad as well. It is really a three step process. As they explain:
First, you must configure the Firebox or XTM device for Mobile VPN with PPTP to allow mobile users to connect. This process creates the end user account, client IP address assignment, and policy for PPTP access. To configure Mobile VPN with PPTP on any Firebox or XTM device that uses Fireware XTM v11.x or on a Firebox X Core or Peak device that runs Fireware v10.x:
- From Policy Manager, select Network > Configuration. Click the WINS/DNS tab to configure WINS and DNS servers.
- From Policy Manager select VPN > Mobile VPN > PPTP to configure Mobile VPN with PPTP on the Firebox or XTM device.
- From Policy Manager, add a custom packet filter policy that allows the user group PPTP-Users to access the trusted or optional network.
For detailed instructions to configure Mobile VPN with PPTP on the Firebox or XTM device, see:
After you configure Mobile VPN with PPTP on the Firebox or XTM device, follow the instructions on Apple s web site to create a PPTP Profile on the iPhone or iPod touch. To configure PPTP on the iPhone or iPod touch device:
- Verify the iPhone or iPod touch uses iOS v2.0 or later software.
- Configure PPTP settings on the iPhone or iPod touch device.
- Connect and disconnect the iPhone or iPod touch device as required.
For detailed instruction on how to configure the Apple iPhone or iPod touch, see Apple Knowledge Base article HT1424.
Let me know if you need any more detail. It took me about an hour to figure it all out, but that's because I didn't really have the slightest idea what I was doing. I have also heard that Watchguard is working on an SSL client for the iPad, which is due to be released later this year.