Referring to the UK, 500fpm (climb or descent) is the notified minimum. However this doesn't preclude a rate that doesn't acchieve this minima, merely that the pilot is required to report to ATC if they are unable to maintain the minima. I think that aewaite17 may have it spot on.
ATC Watcher - I'm no pilot, so I wouldn't guess as to the actual input and required rate. I merely meant that the source I used (the same as Lord Spandex i'm guessing!) allows the pilot all the leeway required with any TCAS RA. I can't imagine an RA would require +8000fpm though!
pl