I also use AT&T, but I bought the N900 knowing that almost anywhere I need high speed I would have it through Wifi. Home, work, and all the local restaurants have wifi for customers, so why would I want to pay an extra $30/month for it? For most things I do outside of that (IM and maybe light browsing) Edge is slow, but quite usable.

If you really want 3G, look into TMoble in your area? N900 supports 3G in their network.