I was hoping a regular Mac user would happen along and take a stab at this.
As far as I know flash drives should work cross-platform, so you'll only need one partition for Mac and PC (I said "should" becasue some versions of OS X apparently refuse to read them at all from time to time -- my daghter ran into this on Tiger, but it has cleared itself up, apparently, so I don't know what the actual cause was, just that it wasn't isolated). I don't think, though, that there is any way to effectively make a partition read-only on a flash drive, nor have I ever actually tried to partition one, though I don't have any reason to believe it can't be done. Optical media is read only by its nature, but flash drives are writeable, and a user would be able to change the properties themselves.
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There are a couple of different ways to format flash drives. I think biggest disticntion is they are permitted to not have a partition table, though they can have one (and can therefore have multiple partitions). I've seen some devices behave differently (i.e. not work) depending on whether the drive was 1-partition (with a partition table) or no partition table. (I can't remember which one worked and which didn't).
As for read-only, well, there are probably tricks, and it's a question of how hard an attacker would have to try to succeed...