This is not really what you were asking for but maybe sell a Level/Quest editor which people can use to make their
own levels and quests that they can share which will help you add content for people whom just want to play the game, but also for people whom want to create content for the game and you would not have to worry about people
sharing levels because it would be a good thing.

Just an idea off the top of my head. Also if it is an applet or something could you just "enable" the level on their account if they pay for it?

Interesting one. I'm hoping to sell these for the desktop too - which makes the backend just a bit more fiddly.

Kev

You could build the packs into the game and just have the payed stuff be account based. That way if you buy them you can play them on your PC, iPhone, etc. Kind of like what endolf suggested. You would have to find a way to keep people from cracking the levels though...

require internet connectivity
check for purchased material
bake/bury the name/account into each package release.
if it gets pirated a little, enjoy free word of mouth/advertising

only have it cost money on mobile platforms and places where its harder to pirate. and places like traditional PC where its very easy to manipulate/hack, offer it for free or just make a fake drm to scare away the low level newbs from trying. 

Offer exe encrypted versions to pc, and although mac/linux may have the direct jar or whatever, its a much smaller crowd and the pc pirates may not think to try and get a hold of the mac/linux jars?