That's a good question, Louis, about what kind of systems open source software is powerful enough to handle. However, I think that since most open source developers build their systems on the systems that they have lying around (except in a PHew isolated cases where the principle objective is to start a cult -- but those cases are Phew and PHar between) the solution for what you are looking for may be difficult to recognize in this space. It seems that the open source OS's are proven architectures on massive system (I'm thinking specifically of Yahoo and Hotmail) but it seems like your requirements might even max those guys out.
I imagine that if someone made a couple or three *real* mainframes available to the OSS community there would be someone willing to build appropriate software for the system or make sure available software can scale. Maybe there are some people doing that right now. But it seems that endeavors like IBM's $1 billion dollar commitment to OSS might initiate something like this, although I imagine it's doubtful any of that money will go to competing products like PG.
As an aside and in response to someone championing OSS, I'm also on another mailing list for non-profits interested in OSS. Recently, someone posted the article where IBM tested DB2 on Windows, Linus and other OSes. Linux came out on top. The test was conducted on some fairly serious hardware, 8 and 16 way servers. For some reason this guy thought that non-profit tech managers would be interested.
If a non-profit ever comes to me, says it has a spare 16 way server and would I be interested in building a system for them, I think my answer would be, "Sure, let's meet. I'm scheduled to get back from my Space Shuttle mission at noon on Friday, but I can just have the pilot drop me off at your offices..."