Looks like Sony will be placing size limits on PlayStation Vita 3G downloads. In an interview with Japanese news outlet ASCII, a Sony rep revealed that at launch, downloads made through the device's 3G connection will be limited to 20MB in size. If you want to downloading anything of significant size -- like full games -- then you're going to have to find a Wi-Fi connection.
The representative did note, however, that this would only affect the Vita at launch, and the size limit could change in the future. Download size restrictions aside, users will still be able to access the PlayStation Store on either a Wi-Fi or 3G connection.
Regarding concerns about the 3G's 128Kbps download and 64 Kbps upload speeds being too slow for certain games, the rep stated that Sony would be providing game studios with "support" to ensure that technical limitations aren't an issue.