I do not think this really belong to "ROM hacking", but not knowing where to post it in, I will post it here.
Retrun-Sonic is a packer/unpacker for games made with The Taxman's Retro-Engine SDK (RSDK), which means, yes, that you can now
unpack the datafile of the new Sonic CD remake, modify the contents and then make a new datafile with the new contents, thereby modifying the game data.
Happy hacking!
http://unhaut.fav.cc/retrunSpread this.
NEW (2013 - 05 - 18): release 1.1