I got a lot of rejected shares too, but I noticed that the number of those fits my usual 2% + the solo shares perfectly (though I'm not sure if it's really the reason for that).

My settings for phoenix 1.50 are:
-k phatk PLATFORM=0 DEVICE=0 VECTORS BFI_INT AGGRESSION=12 WORKSIZE=128 FASTLOOP=false

I just started my miner again and will see if it works now.

With regards to the work queue empty problem:
Try adding -q 3 before the -k switch in phoenix to have more than only one queue. You probably have to figure out which value is perfect for you .

Edit:
Seems to work perfectly now and I'm back to my usual 2%.

I just looked at the deepbit stats page and the last 50 blocks were found within about 18 minutes each on average. I too make less money here, but it's due to my rejected shares which I get on this pool only (other pools with the exact same settings will only reject about 2-3%, not 50-70%).

This doesn't mean I'm mining at deepbit though and I really hope you can fix it as no matter what settings I use your pool rejects so many shares :-/.

Somehow I get about 50-75% rejected shares when I use your pool. Right now it says "6 / 17    40.56 Mhash/s" on your website while I'm getting about 220 MHash/s on any other pool 

Your hash rate calculation seems quite a bit off to me .
It shows 6 MHashes/sec while my miner shows 220 Mhashes/sec, and I get my shares accepted at the rate I usually have on other pools, so it's probably not bad luck .

I wouldn't recommend building a mining rig right now, except if you want to use the components for computer games. If you factor in electricity costs and the increasing difficulty, it will take a long time until you break even. You will get about 10 BTC until the difficulty increases, after that you will get less (I'd guesstimate it to be about 30-40% every 2 weeks). So without electrity costs and a stable price of 16 USD per BTC you might be able to break even in 4 months.

I have to pay about 0,35 USD per kW/h (after converting it from my local currency). Your system will probably use about 800W, which would be about 200 USD for me in just one month. So with this rig I would lose about 50% of my BTC to the electricity bill in the first month, and even more in the following months due to the increasing difficulty.

So expect it to be at least 9 months (probably even more or never) with your rig running 24/7 until you might break even.

I guess it's the missing SDK, too. Try one up to 2.3, otherwise you might just try to use the phatk kernel anyway. To do this, use -k phatk instead of -k poclbm.

I finally managed to lower the memory clock on my card using ATI Tray Tool! 

Also I tweaked air flow a bit and I am down to 72°C now with 1028/425 Mhz on stock voltage with stock cooler, also getting about 270MHashes/sec. Seems running fine too for about 30 minutes now, will see if it stays this way.

Try playing with the AGGRESSION value and add FASTLOOP=false for the higher levels - those might help.

I am doing this and use two different profiles for pure mining and mining while gaming respectively.

I'm using the phoenix miner with the phatk kernel and setting the AGGRESSION parameter to 3 I can fluently play any of my games on my HD6850 and still get about 100-110 MHashes/s.
For pure mining I set AGGRESSION to 16 which after trying differing settings yields the most (about 262) MHashes/s for me.

As for CPU mining, I let the ufasoft miner run at 100% and low priority and got no problems so far (and an additional 13 MHashes/s while my PC is otherwise idle).

Oh I don't care too much about increasing the MHashes/s but reducing the temperature would probably be good as it's at the 91°C right now. At least I'm not the only one with this problem.

I got a new HD6850 by Sapphire and tried to OC the GPU while lowering the the memory clock, as was suggested by a thread in the Mining forum.

I tried both MSI Afterburner and Trixx and still I can't really lower the memory clock of my graphics card. Both tools allow me to set a lower clockrate but the memory clock seems to be stuck at 1000 MHz anyway (at least that's what the graphs are showing; temperature and current aren't dropping either).

Does anyone have a similar experience or is it just impossible to lower the memory clock on this card?