is the stem aluminum or steel? i ask because if its aluminum (and i'll assume the fork's steertube is steel), it may be even more difficult because the metals may have actually chemically fused (i've heard there are chemicals that can help reduce this bond, but have no experience--anyone?)
also, is the wedge free?
as one of the last resorts (before cutting anything, the ultimate last resort)
i've also had luck this way,
1. take the stem bolt out.
2. turn the bike upside down
3. put the stem bolt in from the bottom, so it sticks out of the crown.
4. whack with hammer
-you will probably have to sacrifice the stem bolt
if that doesn't work, you can also stick the stem in a vice, then use a 2x4 in the fork blades to twist.
-i've bent forks this way.
your plan of attack should involve an ever increasing level of things you want to sacrifice.
for instance, try to do it without sacrificing parts, then move on to a method that may sacrifice the stem bolt (cheap, easy to replace), then a method that may sacrifice the stem, then at last resort, the fork.
moral of the story - use grease