Forgive me a noobie question, but I've never actually had a mobile phone before.

Is there a persuasive augment not to buy my N900 with (say) an Orange contract? It seems to be 40% cheaper if I do. I realise it is only 40% cheaper at purchase, and that they will make up their money with my call charges. However, I will need a contract from some carrier or other, won't I? Some carrier will get this money. As far as I can see they don't seem to offer a cheaper plan if you don't take the discounted phone from them. (Or have I missed this possibility?)

So, why shouldn't I take the discounted phone with the contract? Is it just the 12 or 24 month tie-in that is seen as so onerous?

Or is there some technical reason why I shouldn't? i.e. Will the red-pill option still be possible if I take an Orange N900? Will they force me to use their webproxy? Place ads in my browser? Fill my flash with cruft?