For UK users who wouldn't touch a contract with the proverbial barge pole and don't care about cellular voice, the Vodafone TopUp and Go (available for less from other places) is an interesting option. AFAIK you can't buy just the SIM without a dongle, but it's reasonably priced and 1GB can last a really long time on the N900. As a bonus, the dongle works fine with Linux (although you might as well just use the N900 for that).

The only glitch is that the N900 misconfigures the GPRS settings, but after editing the connection and setting the access point name to "pp.internet" it works fine :-)