http://www.gracenote.com/corporate/mp3pro.html

Gracenote is the company behind CDDB and is offering a free software
development kit to non-commercial developers of audio player software
including a royalty-free license for mp3PRO decoding and MP3 encoding. (Thanks for correction, gristus!)

Commercial developers (i.e. selling their products) still are allowed to
use mp3PRO decoding in their software without paying royalties.

Hope I got that right... I would like to see a better Winamp plugin
than the one from Coding Technologies with online buffering capabilities
like the standard MP3 plugin and no playlist timing bugs.

By the way, I found this information in the URL section of another nice web forum:
http://www.mp3pro.com/forum/mp3pro/default.asp

Another quick note: commercial developers only gain royalty free MP3/MP3PRO decoding if they include a free MP3PRO encoder trial in their software.

Right, but not that disturbing, in my opinion. For example, if hardware producers want to integrate mp3PRO into their portable MP3 players, they probably wouldn't care about that condition, because they would have to bundle it with some kind of mp3PRO encoding software anyhow. I would also like to see a Philips Expanium with mp3PRO playback capability, by the way. That would complement its AAC format nicely... B)

Furthermore a free distribution of the SDK for a mp3PRO decoder might help to speed up the decoding process a little bit which is important for hardware portables and internet purposes (download size, online listening). Right now the decoder increases the CPU load up to 3x compared to a MP3 file with the same bitrate because of SBR. Of course this is inherent in the mp3PRO format, but maybe someone like the Mpxplay developer could do some miracles again (like he did with Ogg Vorbis)...

Do not forget that SDK doesn't mean source code. It might just be a static library to link with, as it is the case for the VQF sdk.