Hey, I opened my DIR-615/H2 today, right after I got it in the mail and took some photos.
I've lifted the thermal dissipation pad from the SOC. You can check out my photos on my Google+ account.
Here's one of the pictures:
As you can see, it's a Ralink RT3352F
The Wiki page of the device shows only pictures with the thermal pad in place, so I thought maybe this is interesting to someone.
The unpopulated pad "U34" is interesting I think, as well as the pads for "J4", which I assume is RS-232. No idea if it is possible to extend the RAM, here's the datasheet to the M12L2561616A 32MB SDRAM chip. The components around that seem all to be populated, so it just might be possible (I son't see how to increase the flash size, as it is on the SOC, but 64MB RAM would be quite remarkable, I'd say!).
Unfortunately, I cannot find the datasheet to the RT3352 SOC, only this info sheet, mostly intended for marketing.
As I understand it, the H2 revision uses the same SOC as the H1 revision, so I'd guess it's only small differences.
Since OpenWrt is already released for the H1 revision, I'd assume porting it to the H2 version shouldn't me much of a problem.