I'm fairly sure the Vodafone PAYG dongle SIMs don't allow you to send text messages. You can receive them OK, of course, but then that doesn't require any billing.

It would be interesting to pair a data only SIM with a VOIP service that provides a normal phone number, but it seems a bit risky - you'd only have voice calling when you had a decent 3G connection, whereas a normal SIM will get you basic GSM voice even in quite poor reception areas.

If it were me, I'd go for the data-only SIM if the N900 was only going to be a computer, and I was going to keep another phone for phone type stuff, but I'm not.