If you’ve recently upgraded to WordPress 2.1 (or 2.1.2), you’ll notice that TinyMCE has stopped allowing <div> tags. This is unfortunate, because they’re the way of the future. Out with the <table>, in with the <div>.
Anyway our content management system (era//404 CMS) uses TinyMCE, and the best way around this <div> debacle is to just swap line 25 of the tiny_mce_config.php file (located in the js/tinymce/plugins directory) with the following line (make sure it’s only one line with no break):
Et voila! Your <divs> have returned. I’d obviously recommend saving a back-up of the original config file, just in case you don’t replace the line correctly, but that should do it.