Counterpath recently added support for iOS 4 multitasking and multiple SIP accounts to their mobile Bria application. We currently have multiple people in the office extensively running the app on their phones, and it's been working great—definitely something we will mention to our customers looking for a mobile solution. Check out our update to our review of the Bria SIP app for more information.
So far I know of two SIP applications that include UDP based signaling configurations for background support: Media5-Fone, and now, version 1.1.0 of the Bria. Be sure to shoot us a note if you happen to know of any others.