I don't think you can do that either at the moment.
It only uses the native resolution of the screens right now. e.g. 3 x 24" screens will ONLY work in 1920x1200 x 3 = 5760x1200. I'm sure we're all hoping drivers will be updated to allows different resolutions and orientations, as well as cool things like bezel correction.
Ah, fair enough
I doubt that's an active converter then. The actives are normally at least £60.
I'm not 100% sure it is, but I think it is. I looked into it a couple of weeks ago and the reason for the active converters cost (and USB cable) is that there is a fair bit of circuitry involved in actually converting a DP signal to DVI, mainly in the TMDS.
On the other hand I believe it's STMicro that make a DP to VGA converter chip, it's not staggeringly expensive and doesn't require anything other than the power line present in the DP standard.
On the active/passive argument, the reason you can get passive DP to DVI converters is an addition to the DP spec where it can output a DVI/HDMI style signal, and these do work on the 5870 but due to only having 2 TMDS circuits they don't work whilst also using the 2 DVI's as they need the TMDS so the DP reverts to only DP standard.
However the DP standard doesn't appear to have the ability to output VGA signals (kinda makes sense

But as I said, not 100% sure, not heard of anyone trying it, however I do think after looking at the standards/specs that it should work...