Update 22/7/2010: high register/hex number confusion cured
Although I generally like gedit (GNOME Editor), I was disappointed to discover that it doesn't have syntax highlighting for x86 assembler. It turns out that adding a new syntax highlighter is as simple as writing a little xml file and putting it in GTKSourceView's directory. On my Ubuntu system this is /usr/share/gtksourceview-2.0/language-specs/.
Here's the file in case you're interested.