SAN FRANCISCO – Apple has increased the cap for downloads over 3G networks on the iPhone from 10MB to 20MB, in what some think is a sign of heavier multimedia content appearing for the upcoming iPad.

By now, every time you wanted to download a podcast or application from the App Store that was over 10 MB, you got an annoying message asking you to use Wi-Fi because the file was too large. But not anymore.

Apple lifted the 10MB cap, only to double it to 20MB, which should be a more reasonable figure for video and podcast downloads when out of Wi-Fi range. If you try to download files over 20MB though, a message will prompt you to switch to Wi-Fi.

Besides giving iPhone users a wider variety of content they can download over 3G networks, the cap increase may be a sign that the iPad will require larger file downloads over 3G because of higher resolution graphics and more complex applications.

The initial 10MB cap was put in place so that iPhone downloads won’t put too much strain on wireless networks, such as AT&T, which is . For example, AT&T iPhone tethering, which would produce even more traffic on the network, hasn’t been adopted yet, despite operators around the world enabling the option.

But despite difficulties with the AT&T network, Apple decided to forge a deal for 3G connectivity with the carrier again, this time for the 3G-enabled iPads arriving late this spring. Also, Sling Player 3G streaming has been enabled on the iPhone this week, after AT&T allegedly held back this feature because of fears of increased traffic.

AT&T must be under pressure to keep up with Apple’s traffic-demanding products because the carrier’s exclusivity with the iPhone in the U.S. is soon coming to a close.