Massive Mountain Lion memory leak

Question

I will start describing the problem where I first discovered it.
My early 2011 MBP had been asleep, and upon opening and waking it, it was incredibly slow. I opened activity monitor and couldn't believe my eyes.
I have 8 GB of RAM, and all but 8 mb was in use. Around 6 GB was "inactive". I had no applications running besides Activity Monitor.
I opened terminal and ran the purge command After a short wait, total memory usage was back to around 2 GB. Then right before my eyes, over approximately 30 seconds, the "inactive" memory grew until once again, I had about 8 mb of RAM free. This fluctuated a few mb, but nothing significant.
After rebooting, I opened Activity monitor again, to watch ram usage. Usage increased to a little more than 2 GB. I then launched the App Store. Before putting my laptop to sleep earlier, I had been downloading a 10 GB update to Borderlands, but had paused the download, and quit the application before closing the laptop. I hit resume download, and went back to Activity Monitor. Memory usage seemed normal for several seconds, but shortly started increasing rapidly again. I imediately hit "pause download" in the App Store. But ram usage continued rising, so I quit the application. It kept rising, until my full 8 GB was in use.
At this point I took a screenshot:
The only thing I have left to tell you is that before upgrading to ML, I had previously attempted to download the same update, but hadn't had time to download the full 10 GB, so had cancelled the update. That was in Lion 10.7.4, with 4 GB of RAM, and I had no issues.

Answer 1

Inactive memory is frequently used as an I/O cache.  While I do not know for sure, I think that a lot of the inactive memory may have data that still needs to be synced back to disk (as in disk writes that are cached, and still need to be written to disk).  When other processes need RAM, and inactive memory is used to satisfy the need, the app may nave to wait for slow disk I/O flushing inactive pages before the inactive memory can be given to the requesting app.
Again, I do not know for a fact that Mac OS X keeps cached writes in inactive memory for extended periods of time, however, the slow performance when needing RAM and inactive is all that is available, and considering that laptops consume a lot of power keeping the rotating disk drive spinning, and there are some tasks that update logs on a regular bases that would tend to force a laptop to keep writting to disk, it seems to me that maybe Mac OS X addressed this situation by deferring writes to disk, and using inactive memory to cache it.
The purge command seems to forces Mac OS X to move pages from inactive memory to the free list, and in doing that it would need to flush cached writes to disk, which is why it could take a long time for a purge to complete.
All speculation on my part.
As to Activity Monitor being your ONLY running process, you need to look at ALL PROCESSES in Activity Monitor, as Mac OS X ALWAYS has dozens of background apps running, and if one of them is running away doing lots of I/O, or maybe you have a backup running which will also do lots of I/O, all of which will tend to fill the inactive memory with cached I/O buffers, this could be the source of your growing inactive memory.