Alright, supposing I want to change the player character into a creature (say, a rat) how would I go about doing this?

I know of two mods that do something like this:

http://www.tesnexus.com/downloads/file.php?id=5814

http://www.tesnexus.com/downloads/file.php?id=28563

Thanks Pseron Wyrd. I'll take a look.

One of the things that crossed my mind though. Couldn't I get away with mounting a character on a creature and then making you (the rider) invisible?

This is what I do and with the exception of how base Controls are altered for being mounted its a far superior method its also the only method that addresses 1st person view as it relates to creatures -> just mounting and using some fairly basic Mounted Combat scripting is all that is required -> OBSE is almost a must though you can get Mounted Combat to work without it just fine.

Yes, and that is how xilverbullet does it for his polymorph eagle, wolf, tiger spells in Midas Magic.

Yeah, I'm hoping to be able to accomplish the minimum without going to OBSE.

So the idea is something like this:

1.Create actor copy
2.Cast (invisibility)
3.Move (actor) to (creature)
4.Activate (actor mounts creature)
5.Is Riding Horse (condition to start timer)
6.Script effect elapsed seconds (condition to return to regular form)
7.Activate (dismount)
8.Move (actor) to (actor copy)
9.Delete actor copy
10.Dispel (invisibility)

Just so you follow my reasoning, the player leaves their body behind (1,2) becoming the creature for a set time (3,4,5,6), until time runs out and they return to their regular body (7,8,9,10)