Allow sorts to use more available memory

The recent "data warehouse" thread made me think about how I use
work_mem for some of my big queries. So I tried SET work_mem = '4GB'
for a session and got

ERROR: 4194304 is outside the valid range for parameter "work_mem" (64
.. 2097151)

A bit of searching turned up the "Allow sorts to use more available
memory" section of the to-do list. Am I correct in reading that the
max_val is 2GB and regardless of how much RAM I have in the box I'm
stuck with only using 2GB? Am I missing something?

I'm using: PostgreSQL 9.0.4, compiled by Visual C++ build 1500, 64-bit
Windows 2008 Server Enterprise

Thanks,
Bob