Jonathan Zdziarski author of the book iPhone Forensics found out that Apple has apparently a method to remotely deactivate iPhone Apps on your iPhone.
Apple is apparently keeping a live blacklist of bad iPhone Apps. The list is empty right now.
On the one hand this feature is pretty cool and allows Apple to have a lot of control over iPhone applications. So in case someone sneaks an iPhone App with a Trojan horse inside onto the iTunes App store, Apple still has the ability to kill the app, even if you installed it already.
On the other hand if Apple is using this capability on applications that they just do not like (e.g. NetShare) it becomes a bit scary.
More details on MacRumors and iPhone Atlas. Update: See also this clarification about the blacklist only applying to protecting Core APIs.