Yes i am using the OpenCL miner. Before i was using some old ATI radeon card and was only getting 7Mhash/s. Any ideas?
  I am going to remove all the related drivers and try again but please let me know if you have any other potentially useful info.

Sorry to be posting in this category but im not allowed to post anywhere else. GUI miner was working fine untill yesterday when i upgraded my graphics card to a  XFX radeon HD 6750. I have installed all the drivers but since then GUI miner reports a 140Mhash/s 'speed' but in about 8 hours i have had no accepted or stale shares. Any ideas?

I am having a similar issue. I had a very basic GPU that was mining at 7.5 M/hash and yesterday i upgraded to an xfx radeon HD 6750 and GUI miner is reporting a speed of around 140 m/hash but in 6 hours i have no shares accepted. Any ideas?