if you've had your XT connection live and connected since before you went in the store, then it should be easy to coinvince them that your intention was to purchase a compatible phone.... and they should have had enough clue to ask the question and provide you with the right model. I'd say go back and see how it goes.

Just cause a couple of months has passed doesnt mean that if they sold you something that doesnt work 'as described', that they can get out of righting the mistake...

does your invoice say "Vodafone", or just i9000? How would joe bloggs with zero clue necessarily look at the pre-order and know the difference?