On Dromaeo, performing the test causes Safari's memory usage to climb through the first few tests, increasing by around 80MB from 144 to 220 MB (this seems normal), then suddenly increase by around 600MB to 830MB right before it starts to run the "DOM Modification (Prototype)" test. The memory usage then stays constant for the remainder of the test. Running the test a second time produces barely any increase, but closing the Dromaeo tab does not cause the memory usage to decrease.
The values quoted were derived from Process Explorer's "Private Bytes" measurement.
For comparison, Firefox nightly never exceeds 300MB of usage throughout the test, and ends up with the same memory usage as before the test. It rises rapidly followed by rapid decrease, presumably due to periodic garbage collection.
I don't see this problem in the Mac with r39469 on 10.5.5 - RPRVT doesn't get much larger than ~200Mb, with Real Memory getting to about ~400Mb. I certainly don't see a problem with the "DOM Modification (Prototype)". However, the memory that is used does persist after all tests are run until the window is closed.