Joining AT&T (T) and T-Mobile (ahem), Sprint (S) will now allow Android users to buy Android Market items and have them charged to their monthly Sprint bill.  This theoretically simplifies purchasing and removes the barrier to entry for paid Apps for some Android users.  Google remarked:

We believe that Direct Carrier Billing is a key payment option because it lets users purchase and pay for apps more easily. It’s also important because it offers a convenient way to buy in regions where credit cards are less common.

Google (GOOG) currently only has carrier billing in the U.S. and in Japan, both regions where credit cards are pretty common so it seems like there are imminent plans to roll this out to carriers in countries where credit cards aren’t as common.

Sprint last month announced that it would offer Google Voice to any of its post paid consumer voice plans and seems to be cozying up to Google in a big way, especially since the AT&T-T-Mobile merger.  Sprint also announced it would carry its own 4G version of the Google Nexus S smartphone.

Sprint would be the only major U.S. mobile carrier not to carry Apple’s (AAPL) iPhone should the merger go through.

The only other carrier without direct Android billing options is Verizon (VZ) and since they have their own VCast App Store, which charges to the phone bill, I wouldn’t expect them to be signing up any time soon.

The new billing options should reach Sprint customers within the next few days.  It won’t affect Sprint’s pre-paid subsidiaries like Virgin and Boost.

More on Fortune:

• Sprint teams up with Google Voice
• AT&T-Mobile: AT&T buys T-Mobile USA for $39B
• Virgin’s unlimited data phone plan: only $25/month