I've noticed a similar problem on my machine for a HCMD unit.

16/10/2010 17:05:39|World Community Grid|Started download of 88bb1c56c98897c76e6075586559ead3.pdb.gzb
16/10/2010 17:05:41||Project communication failed: attempting access to reference site
16/10/2010 17:05:41|World Community Grid|Temporarily failed download of 88bb1c56c98897c76e6075586559ead3.pdb.gzb: HTTP error
16/10/2010 17:05:41|World Community Grid|Backing off 1 min 0 sec on download of 88bb1c56c98897c76e6075586559ead3.pdb.gzb
16/10/2010 17:05:43||Internet access OK - project servers may be temporarily down.

And so on..............

This stopped any other units being downloaded. I did a manual update to send my completed work back but no new work arrived despite being out of work, only this one HCMD unit was continually being "downloaded". (I first noticed this yesterday but left if to see if it would sort itself out as I had other units to process). As mentioned above, I tried a Project Reset but this did not make any difference as this unit still stayed in a permanent downloading/backing off status.

I have managed to abort this unit to get it out of my list and I have now been able to get some new work. Hopefully all is back to normal now.