I usually use WIFI, but I cannot always be in a WIFI area and I don't want to lose out on the whole iPad experience while away from hotspots.

I won't be doing anything like streaming audio or video on 3G, but will 250MB per month be enough to sync and check multiple email accounts, receive push notifications from various apps, do Google Local searches in Navigon and receive traffic updates in Navigon and use some apps like Yelp for an hour a day every day?

I think it depends upon the size of any attachments that you'll receive on email. Those 250 MB plans can be upgraded, so you're probably good to go and then see whether it works out. I know that there are relatively few unlimited plans in the US any longer, so I'd try 250MB. One way you could track this beforehand is to see how much WiFi data you use when you don't have 3G. There are a couple of apps out there that independently track WiFi and 3G data, such as XSysInfo, and you could try to see how much data you would be using that way.

Tim