64 bit Postgres 9.1 Limitions on Windows 2008 R2?
No clue.
What kind of scale (up or out)? From what little that I've found on the web, it appears that Pg on Linux can handle more connections than on Windows.
Not if you are running anti-virus software on the Windows box ;^). There is an old (8.2 vintage) pgsql-performance thread here regarding performance differences. The take away seems to be "it depends" (based on usage) and that you need to do benchmarking to really know how much performance difference there will be. There is also some discussion regarding performance differences on serverfault.