I hva a tmobile blackberry 8700.

It says on the about screen that I have V4.10.284.

I have heard rumors of a new os. Is it out yet?

How do I get it?

4.2 is not out yet for the 8700. A list of the latest versions for many BlackBerrys can be found at Latest Operating System Available For Each Blackberry Device

When will 4.2 be out. Will my 8700 be upgradable?

What will the difference be in 4.2?

No one knows.
Yes.
Major difference is additional media support (it will work with the media manager in Desktop Manager 4.2).

plus for me a 'minor but a biggie' - there's a separate icon/"mailbox" on the home screen for text messages, hence no text messages get 'lost' in your general mailbox. Previously I was finding that I could receive a text then 20 emails and then never spot the 'lost text'

plus I like mp3 ringtones..

MP3 ringtones are currently a feature on 4.1 on the 8700...