I recently downloaded Outpost 3.0 and let the AntiSpyware scan do its thing. Once it kicked off I noticed that McAfees McShield was running. I can only assume that they are using McAfees engine. As for the definitions I dont know. McAfee is from what I understand a top notch Antivirus Antispyware product.
Outpost's Anti-Spyware plugin uses Agnitum's Tauscan engine (see Agnitum Launches First Beta Version of Outpost Antispyware). However any program that scans files will cause most installed anti-virus software to jump in and check each file also - this is why having multiple real-time scanners can be a bad idea.