I have a samsung tocco touchscreen phone and bought some screen protectors on Ebay. They are excellent and the screen sensitivity was not reduced at all. Sorry but cannot remember the seller offhand, but they are so cheap I would definitely reccommend that you try some.
click here these are the ones I use for my phone and they are £1.49 (inc. p&p) for 6. They are excellent and do not reduce sensitivity. I laos bought a leather hoder for 32.50 inc p&p. Don't bother using shops, Ebay is way cheaper.
Fair enough if you have a plastic touch screen - I use an iPhone3GS, and don't see the point in putting a screen protector on it, because 1. The official ones I believe cost £10 (lol) in an apple store, for a piece of sticky plastic. 2. It's a glass screen, and scratching glass isn't easy. 3. What's the point in having a smear resistant oleophobic screen, if you then stick a non smear resistant cover on top of it.