I was experiencing quite fast battery drainage as well after installing iOS 4.2.1.
I plugged my iPad to my desktop and iTunes last night, and clicked the restore button to make a "clean" install of iOS 4.2.1.
I was quite dubious about what the results would be, but I have to say that to my surprise it does really seem like my battery drainage is now back to what it was before. I need to use it a bit more to be absoultely sure, but it does really look like it!!
So everyone who is thinking their battery is running out really fast, I would highly recommend trying this!