Here is a working example:
for /f "tokens=1* delims=" %%s in ('gnuwin32\date +"%%Y%%m%%d_%%H%%M"') do echo output_%%s.log > output_20110215_1057.log
Works in Windows XP, Vista, Windows 7.
Remember:
1) The variable used in a for loop must be a one letter (at least in XP, Vista)
2) gnuwin32 is a set of GNU tools like grep/sed/awk/date which can be run natively in Windows without cost of additonal Cygwin-like layers