I just had this problem myself.
It seems like a commercial solution is the best. I am converting satelight imagery to proxy vegitation, so I was looking for a scatter method that used grayscale maps to distribute the trees. I just bought the latest 64 bit 6 core intel machine with 2 GB video memory and 12 Gigs of ram.
The best free method I could find was using partacle flow and an instance coverter script from Bobo. This method was still bringing my computer to its knees after exceeding 200,000 mr proxy trees. I caved in and finally bought Carbonscatter which has free render nodes. The UI was a bit hard to figure out at first, but I was able to find some Vue tutorials which used the same UI and mapping techniques. Carbonscatter recognizes easynat trees which come on the 2011 DVD or can be downloaded for free.
Good Luck.