To answer your question, in the US they've offered a default of 256, which is better than the usual 128 by iTunes for the same price.

Google for "amazon mp3 bitrate" and the first hit will contain the answer to your questions.

Amazon USA sells it's songs encoded at 256 kbps VBR. There is no reason for changing this policy over here.

And what do Amazon customers get? Are these MP3 files with only 128 kbit/s or is Amazon perhaps going to offer MP3 files encoded with **variable** bit rate (VBR) up to 320 kbit/s? The BBC offered a while ago classical music for free. The bit rate (and quality) was so low that I decided not to download anything from the BBC even at no charge.

MP3 is not MP3. Any word on the quality of the MP3 files would be nice. If main features are not advertised, then assume it is rotten.

For an excellent MP3 encoder, take a look at Lame. Lame supports VBR and comes with many other features. Making MP3 files is one thing, making them real good is another.
http://lame.sourceforge.net/