If the iPhone visual voicemail feature is the one thing you're most looking forward to, then check out callwave. By redirecting your voicemail to their free voicemail service, you can access any of your voicemails at any time (from your computer). There's even a widget version (OS X and Yahoo) that lets you hear messages in any order you like.
Along with that, Callwave also sends you a text message and an email telling you you've missed a call. Not quite the Visual Voicemail of the iPhone, but cool for its desktop integration. When we tried it, the audio quality was decent enough to use as a full-time voicemail solution.