I bought Inesoft Phone 5 a few weeks ago and it does what your looking for, although it's not free... It also has a bunch of other things that go along with it like a dialer, contacts, ect but the ones you dont want to use can be disabled. Not really worth the $15 if all u want is the caller ID IMO though...
Website is
http://www.inesoft.com/eng/index.php?in=phone.html if you want to check it out and try the trial... if you do set the caller ID skin to Standard QVGA for the one with slide to answer/ignore