Designing for handlhelds is always and interesting topic. Much depends on the software the handheld is using, but since the operating systems and browsers most handhelds run are so much better than they were a couple of years ago you can get slightly more adventurous.
Style your site with CSS for the best possible chance of a good result on a PDA.
Almost all PDA's and phones will ignore the CSS and run the page content as basic (X)HTML code. More complex (X)HTML tags will be mangled and can produce some unexpected output, usually in the form of gobbledegook characters inserted into page text. Testing will highlight these problems and give you the chance to overcome them.
Considerate layouts and careful use of images can help and you can, if you like, go for an all out, full on PDA compatible site.
The MSDN has a useful introduction to the overall concepts involved: click here
Some Googling will also give you more infromation than you probably want.