As title. Of course using Android market and the Google Checkout payment. By the way, how about other market-like apps?
First be aware that you're transferring data over a wide area wireless device (the range of 3G is much wider than Wifi), therefore it is easier for an attacker to snoop on whatever your device is transmitting on 3G than on Wifi. The 3G/GPRS only has a lightweight encryption (to limit unnecessary use of CPU power and battery, which is already scarce in mobile devices), this encryption should never be relied upon.
However, applications can use its own encryption. I have not checked into it specifically, but I would presume that Google Checkout on Android does use SSL encryption; which is widely accepted to be strong enough for doing online purchases.
In short, you should not need to worry about doing online purchases using Android Market. If you're doing purchases on other apps, you should check whether they're using SSL or other strong encryption. Be aware that your banking details are being transmitted wirelessly, it is trivial for anyone to record data that are transmitted using radio waves; your only line of defense is SSL encryption, which should hold attackers at bay long enough until your credit card expires.