Here are some parameters that I would like to fulfill:

1) My audio is primarily FLAC with some MP3, so transcoding is a must.
2) Should be relatively fast (almost 12000 songs)
3) Support for FLAC replaygain tags
4) "Play all" support

Here are the problems I have encountered:

Microsoft - Flac support is bad
Twonky - Site says it does not support transcoding - this is a showstopper - and if I am paying for something I don't expect that I need to re-architect it to make it work...
TVersity - It is kind of slow and clunky but everything functions except for the replaygain tag support .... So close.

Firefly (a.k.a. mt-daapd) is IMO a wonderful product, there is a ton of support for it and it is lightweight and lightning fast. Don't get me wrong I hate iTunes and iPods for that matter but this seems to be a more useful standard to me than uPnP for open source implementations. Seems ironic that DAAP (born from Apple I think) would be more GPL friendly than something called Plug 'n' Play.

There aren't any DAAP clients for the n800 are there?