The real secret is how many times your device connects to internet to check your mail,twitter and similiar applications because every time when it connects your mobile operator takes off from your account 100kb of data. Even if you have no new messages. But if you have application on your phone which keeping connection all the time your data consumption is about 80% smaller. I´m using Nokia Messaging from a few weeks and it keeps connection all day long so I observe that data consuption is very,very small.

For what it's worth, with my previous smartphones (Nokia E61, E90 and iPhone 3GS), I only used a few dozens of MB per month. That was mostly spent on email, Twitter and a bit of web browsing.

With the N900, I've only had it for 2 weeks but am already at 635MB of downloaded data (and 35MB sent) - that includes both 3G and wifi. Part of this is probably the new toy excitement syndrome. But in general, the N900 is just a fantastic Internet device that pushes you to be online all the time.

IM is always there and so seamless that you just use it without really thinking about it. Web browsing is a real joy. YouTube videos display inline and just work - I've found myself watching much more embedded YouTube videos than I did with my previous phones. The FM transmitter is great too - I've been regularly listening to French radio streamed over 3G while driving thanks to it.

That's a very valid point actually but it depends on your operator.

In the UK, AFAIK, all operators charge by the Kb when in the UK (and if you get an N900, you'd really want to get an unlimited data plan anyway). When roaming, I remember that some operators used to charge a minimum of 100Kb for each connection. Not sure if this practice is still in place.

Some operators give you all you can eat for X minutes. For example, Vodafone NL asks 0,75 EUR for 15 minutes with speed 3600/384 while T-Mobile NL costs by the 10 kB, rounds up, and provides less bandwidth. Vodafone NL would work great if you were to download some Google Maps for offline usage whereas on T-Mobile NL this would be expensive. Conversely, if you'd check your e-mail every hour or stay connected Vodafone NL's scheme would cost you a fortune.

The reason you use 100 kb of data is because of authentication with your e-mail server. If you stay connected some bytes will be used to keep the connection alive, and when it does check for messages it only has to sent that command every X minutes and receive reply hence skipping authentication. This will not work with POP3, but will work with IMAP4, and I suppose as well with something as Nokia Messaging. Also, I think IMAP-IDLE, which is supported on S60 client, reduces the need to check every X minutes further lowering data usage while allowing real-time communication when necessary. I don't know about P-IMAP though.

Here in Italy there are a lot of rate plants depending by your vaforite operators. So ... exactly your data/month use depends on you.
The only thing that I can add is that in N900 you can configure as you want your apn and internet connection, so you can use also a special promotion made by the operators for only one mobile.

Example: Here in Italy you can ave 2 Gb/month ONLY if you have an Iphone. But ... the same SIM works great also into the N900 :-)