Just saw you over at another thread.
Anyway the only thing I see is that MP3s as a listening format are just about OK quality, CDs are the real deal and if you sell any CDs or plan to you should master for the CD.
That said, 128 kbps seem to 'leave' some bits out of the mix. Clashy, bad cymbals is the first thing I can think of, along with lots of compression so dynamic tracks don't compress well - jazz and acoustic music suffer quite heavily. The new age pop and rock (I'm generalising a bit here. so bear with me) seem to survive MP3 compression better than older rock tracks (like Stairway to heaven, older Santana stuff) which get really bad after MP3 conversions.
An example: Comparing 'Put your lights on' and 'Blues for Salvador' after converting to MP3, I find that the latter track sounds really bad but the former, is listenable.
I'm not sure you can master 'differently' for the web and for physical media, rather master for the physical media and convert to MP3.
In web design, it's a different ballgame. Most of the time it's MIDI you hear as background on webpages, as downloading would take way too much time if it was MP3. In case where riffs etc. are in MP3 format they are usually downloadable by clicking.
I'm not sure those are mastered 'for the web' so to speak, but there is some thought. I just don't think there's any real advantage to it unless you were pretty sure you're not going to be using the piece anywhere except the web. And that's pretty rare.
The problem is when you have a dynamic track that breathes a lot. Harvey Reid is one such guy. He's an amazing acoustic guitarist, who offers web downloads of some of his music at two quality levels, one at 128 and the other at 256 so that the user can download better MP3 files if she wishes.
