I could totally be using the term "memory leak" incorrectly.
Basically, levels don't appear to be being unloaded from memory. This is extremely easy to reproduce. It happens even on a clean install of UDK. Not sure if it happens in earlier builds. It pretty much makes our game unshippable though.
Here's the scenario:
Run UDK.exe. Task manager says UDK.exe is using 314MB RAM (or whatever).
"open vctf_necropolis" in console. UDK.exe is now using 945MB.
"open vctf_necropolis". Using 1164MB.
"open vctf_necropolis". 1387MB.
"open vctf_necropolis". 1610MB.
"open vctf_necropolis". 1819MB.
"open vctf_necropolis". 2064MB.
And so on until you crash because you ran out of memory.
This happens with level streaming too. In our game, we use level streaming a lot, and what's happening is every time a level loads, we use more ram, and when it gets unloaded, we don't use less. Eventually we inevitably crash.
I haven't tried this as such, but I suspect that even you had this simple level streaming setup, running in and out of the level streaming volume enough times would cause a crash from lack of memory.
Edit: This still happens on February and March builds. It started on the May build of 2012, so let's everyone drink to most of a year of leaking memory, I guess.
Edit edit: This is fixed in July 2013 UDK. Many thanks, Epic guys.