Yeah, what Joe said regarding graphics
And don't bother with mouseovers, popups or opening things in a new window because most PDAs (well mine at least) won't support any of this.
I'm using a lot of display:none; in the PDA stylesheet I'm working on for ukguest.com to get rid of anything non essential... going with text links, minimal imagery, a smaller version of the logo etc. Fortunately the listings just happened to look fine on PDA with no changes which is a real bonus