Are you generating less than 13.07BTC/day?
Get the latest poclbm miner (2011-04-26) and pass -v -w 128 -f 60 (the -f 60 you might want to tinker with, but on Windows, I had some stability issues after a week or so and it only changed the hash rate by about 3MH/s max, but my machine is used for work as well via VPN). I get roughly 366MH/s running Windows 7 x64 (latest Catalyst drivers and Stream 2.4 (which is probably not required) using a stock MSI Radeon HD 6970 card with no overclocking and have a fan profile setup to keep the temp at 75-78C. I have seen some postings elsewhere of using -w 256, but I am not sure that is helpful or not [going from 64 to 128 certainly was].
I would love to hear how this works out on Ubuntu (or even Gentoo). I suspect you will get the very best performance on a Windows 7 or Vista machine with Aero turned off using the parameters I mention above, mainly because the drivers for Windows are undoubtedly where they spend most of their efforts on improvement.