Early adopters of the handheld in Japan can pay either approximately $12 for 20 hours of 3G connectivity, or approximately $63 for 100 hours. The plus side of this time-based plan is that you can download as much as you want in as little time as possible, and not be plagued by the prospect of extra data charges and caps. You could maybe download a handful of games in an hour, and still have 19 hours left on your plan.
In the US, however, plans are considerably different, and probably more inline with what we should expect in Australia. It's been reported that the provider of 3G connectivity for Vita, AT&T, will offer two plans: $14.99 per month will get them 250MB of data, while $25 a month will provide 2GB of data.