Thanks Xenakios. Wow over on Digg.com there was a link to the following page:
http://daringfireball.net/2007/04/some_facts_about_aac
Very nice article BTW and it does talk about mp4/aac licensing. In addition I found this page with actual license pricing schemes:
http://www.vialicensing.com/Licensin...duct=MPEG-4AAC
When looking around further I found this page:
http://sourceforge.net/projects/faac/
This is the Freeware Advanced Audio Coder. Source code can be found in the above link and .exe files can be found here:
http://www.rarewares.org/aac.html
So it appears that it will cost money to include mp4/aac support in Reaper. However it might be possible to give the user the ability to install this function themself. Anyway, just sharing the info.