The latest App for Iphone is called "Cisco Jabber" (not "cisco Jabber IM"!), you can find it in the App Store. It works with CME 8.6 and above, CLI configuration only. CME 8.6 on UC500 has been delayed (again) until April 10th, and even after this I doubt it will be supported by the SB Support/CCA.
But it should unofficially work of course, it's simply a SIP endpoint.
Follow this guys excellent BLOG (once 8.6 is released for UC) and I'm pretty sure it'll be good to go.
http://www.ccierants.com/2011/05/finally-cisco-mobile-81-supports-cme-86.html
If you need an urgent alternative, you can use "Groundwire" (again in App store, but not Free) - it's a SIP client for Iphone that works pretty flawlessly with the UC500, and the configuration is same as above (although the MAC address can be anything). Just use TCP if you want the backgrounding to work correctly.
p.
