There are numerous stories of KORG MS2000s being bricked by failed OS upgrades - the bricking is reversible though, it seems. Since the upgrade solves an arpeggiator MIDI timing issue, there's good reason to get it done.
I managed to do that without wrecking the synth, but it was some effort nonetheless. My Midi sonic 2x2 interface refuses to send SysEx, while receiving is working as intended. After trying 3 different OSX apps to transmit an ordinary patch SysEx, as well as doing a dump from my ER-1 drum machine, which had the exact same problem. Or, actually I started to suspect that the problem was in fact the MIDI interface. When I tried transmitting a dump the light flashes but the synth shows no sign of receiving anything.
This theory proved correct when I got hold of a Steinberg USB2MIDI / M-Audio MidiSport 2x2 (they're both the same hardware) using M-audio's recent OSX drivers. After a bit of testing and nail-chewing I ran the unofficial 1.07 upgrade - which is nothing more than the SysEx files lifted out of the original Windows-only firmware upgrade app. Instructions are included, and the upgrade itself is comprised of 33 easy steps ;-) To transmit the files I settled for SysEx librarian for OSX, with a transmit speed of around 1900 B/s. The upgrade worked like a charm. First improvement I noticed was that the text displayed when saving a patch had been changed from "Writting…" to "Writing…". Good job!
With a proper MIDI interface I'm also happy to report that KORG's MicroKorg editor (look under software) works! So now it's back to happy synthesis, and I guess I need to get me a better MIDI interface eventually…
By the way: to check your firmware OS version, hold MOD SEQUENCE [On/Off] and  while powering up. The display will then say "My version is XX" meaning you've got version 1.XX.