I'm having the same trouble (though memory clock jumps to 300) under Linux.
Is there an aticonfig command I've missed out? I've enabled overdrive and set the peak memory clock to 300, but the utility reports that I'm running 1050.
I've had this issue with every 6XXX series card I've used in Linux (Ubuntu 10.10).
Using aticonfig I've never been able to set my XFX 1GB 6950s memory clocks less than 125mHz lower than the GPU clock. In order to set memory clocks outside of the default ranges at all, I have to use AMDOverdrivectrl to set the default clocks for each power state, but even those don't 'stick' - as soon as I 'push' the card the memory clock ramps up to max.
The 6870 I picked up recently won't drop below 100mHz less the GPU clock.
It seems to be the same issue as Windows boxen, but at least there the manufacturers have specific utilities to get around it. It also appears that it may be vendor/card version specific, as some people have had success underclocking mem on a handful of these cards in Linux.
With the BTC prices so low and my power usage so high lately, I'm resorting to bios flashing a lower memory clock and voltages on all of these cards this weekend. Totally kills their use in Windows or for gaming (unless you flash them back), but will be worth it for mining - at this point I need to eek out every bit of efficiency I can in order to stay above water.