Here's how I read it...
Rules of the Game

olymorphing - Part 3:
http://www.wizards.com/default.asp?x=dnd/rg/20040525a
<snip>
You can choose any of the following creature types: aberration, animal, dragon, fey, giant, humanoid, magical beast, monstrous humanoid, ooze, plant, or vermin. As with
alter self, there's no limit to the subtypes the subject can assume, so long as the base type is on the list given above.
(SKIPPING AHEAD)
If you're having trouble deciding which type of traits you gain, here's a list of what you get and what you don't.
</snip>
After the above quote it posts a complete list of all types and subtypes and associated traits. You are unable to polymorph into some of the types and subtypes listed.
It's pretty confusing because some of the types and subtyes listed say "You can't become a swarm by changing form." or "Undead creatures are not alive, so you usually can't assume an undead form" which leads you to think you can turn into the types that do not have prohibitive notes listed.