Here's the answer:
Does the iTunes Store for iPhone, iPad, and iPod touch work over a cellular data network?
For all types of available content, the iTunes Store for iPhone, iPad, and iPod touch works over Wi-Fi. If you own an iPhone, iPad Wi-Fi + 3G, or iPad (3rd generation) Wi-Fi + Cellular, you can also use a cellular data network. Cellular downloads are limited to apps, music, music videos, books, audiobooks, podcasts, and iTunes U files with a file size of 50 MB or smaller, depending on the wireless carrier. You cannot download movies or TV shows using a cellular data network, regardless of file size. Fees may be associated with downloading over cellular networks; contact your wireless carrier for details.
The answer comes from here:
iTunes Store: Frequently Asked Questions for iPhone, iPad, and iPod touch