There are quite a few on here that have jailbroken phones and are developing apps for the App Store, no problems (2G and 3G at least, I presume it's the same with 3GS). However, when you say that you'd 'like to use some of the functionalities that Apple left out', I presume you mean undocumented API's? In which case, your app will most likely be rejected...
Welcome btw.
