In the footsteps of Google Mobile, GrandCentral now has a lightweight, mobile-specific version located at grancentral.com/mobile.
Don't get confused: this is not yet a Java app (though I'm hoping they're going that way with it), but rather a very convenient way to access most GrandCentral features on your mobile device.
In order to really use the service effectively, it's best to have a device that has the capability to play MP3 files so that you can listen to voicemail messages -- works great on my Pearl.
One thing that I haven't tested, though it is listed as a feature, is that if you get voicemail files sent to your e-mail and receive them on your mobile device and listen to them on your mobile, using the GC Mobile client will synchronize those activities. Because I don't have my GrandCentral messages forwarded to the e-mail addresses I access on my Pearl, I haven't tested this, but it sounds pretty cool if it does what I think it does.
As I said above, hopefully they'll package this up in an application just like the Google applications on the Blackbeery; it would be very cool to simply have a little GrandCentral icon rather than having to launch the browser.