I had a wacky problem tonight, and with the help of folks in IRC, was able to work it out. I figured I should document it here.
I noticed this evening that I could no longer access the gui of any RP2 machine. Right clicking simply did nothing. I tried all manner of troubleshooting, starting with making sure the configs matched. I tried uninstalling all the mods i had installed recently, but that didn't help either.
I finally realized that I had been keeping my server up to date with the newest mcpc bukkit builds, and was currently at 56. I tried rolling all the way back to 49, and all of a sudden I could access my RP2 guis. IIRC it worked on all of them up to 55. Someone mentioned that forge does some autoassignation of gui ids in certain cases, so i tried updating my client version of forge (which was at the recommended build of 59). I updated all the way to client forge 64. With a client at 64, I could once again see the RP2 GUIs on the server running MCPC 56.
TLDR: If you are going to keep your server version updated, keep the client updated as well.