I didn't notice this for months. I was only able to use ca. 900 MB of my 1024 MB of RAM. It only happened accidentally that I read through my logs and noticed this:
Dec 26 19:33:37 localhost kernel: Warning only 896MB will be used.
Dec 26 19:33:37 localhost kernel: Use a HIGHMEM enabled kernel.
Dec 26 19:33:37 localhost kernel: 896MB LOWMEM available.
After setting the "High Memory Support" kernel option to 4GB and a quick kernel recompile, I can now use my full RAM. Nice, eh?
Update 2005-12-31: Thanks a lot to all the people pointing out (via comments in the blog and emails) that there's a better way to do this!