It's possible to get the handset on a 24-month contract for as little as £20 per month from Orange, though that'll only give you 200 minutes and you'll have to shell out £170 for the handset. A similar 24-month deal from O2 for £30 per month will net you 600 minutes, and the phone itself will only cost £10.
If you're determined to get the handset free, then a 24-month deal from Orange will set you back £35 per month with 900 minutes. The cheapest 18-month contact is £25 per month, again from Orange, with 600 minutes and £220 upfront phone cost.
Are you itching to get your hands on the N900? Tell us why, and which pricing option you reckon you'll go for, in the comments.