For a quick point of reference, I have used 600 MB of data since launch day on my 3G. This means based on my current usage, I will accumulate about 800 MB of data usage in a monthly billing cycle.
I would say I'm about an average data user too. I download occasional apps, browse the web a little each day on-the-go, check sports scores, etc. Nothing out of the ordinary, especially for an iPhone user. However, I rarely have wifi available so you may have an edge over my usage in that regard.
Just thought I'd offer an idea of an "average" iPhoner's data usage for comparison's sake. Good luck, and watch out for those overages!