I don't think I'll be participating in that; I'm not sure what the point is anyway.
It was fun being part of RC5-64, especially the first year or so, and I was pretty enthusiastic and kept my computer running day and night.
But as the contest dragged on and on without showing signs of finishing, I sometimes wondered whether they'd even find the key before the reached 100% -- perhaps the right key was in some packet that someone downloaded but only processed halfway through and so they'd have to re-issue all the keys which didn't return a result.
Anyway, I think one of the goals was to show how quickly keys can be cracked if enough computing power is assembled. I think the more than four years that RC5-64 took shows that even with an enormous array of hardware available, it'll take quite a bit of time. It's not like DES which was cracked in a matter of days by specialised hardware.
And RC5-72 has 256 times as many keys -- even if computers are faster now than they were when RC5-64 started off, and even if computer continue getting faster during the time the contest runs, I think it'll take ages until the find the right key, and they won't have proven anything (except that it's possible to crack codes by brute force, which people know anyway).
What's it that Moore's law says -- processing power doubles every year, or something like that? In that case, computers are 16 times as fast as they were four years ago, so by a bit of off-the-cuff arithmetic they'll be able to crack a key space 256 times as big in only 4.5 * 256 / 16 = 72 years! Yay!
I know I won't be participating in RC5-72.