whoa, last time i sow or used <center> was few good years back)
regarding tables, recently done one paysite tour, sliced in div 100% valid - we had to reslice it all in tables because client's cms had <tr>s and <td>s hardcoded into system - how crazy is that))))
iframe is no good too and default adobe's flash embed is not valid..
its hard to switch when you're used to old methods of doing things, but most of time after changing it becomes much better and easier and sometimes result in more options what you can do.
many do <div><div><div><div><div><div>.... find it crazy - call it transition from past century (tabled nesting), there is really no need to do such complex nesting to keep structure in tact. deep nesting is harder to maintain..