I would be reluctant to do it myself as I repaired my own PSP and have had nothing but problems with dust ingress since. Its simply impossible due to static to replace the screen in my house without a ton of dust stuck to it, your mileage may vary depending on if you have carpets and/or pets in the house, I have both. That said, if they do it in-store, you still have a certain risk there too as generally LCDs (and probably assembling of the phones themselves) are intended to be replaced in a clean room then sealed so that dust cannot get between the screen and the touch screen (or screen cover in the case of the PSP). Although I have doubts if even the repair centres stick to those guidelines as the few times I had things like my laptop repaired, it came back dirty.

It all depends on price I guess. It might cost you double to get it done professionally instead of doing it yourself, but its worth it. Now if its going to cost 3 times or more, then its probably worth risking it yourself. It does appear the N900 screen is easy to replace due to it being separate from the main unit thanks to the keyboard slider.

There is another risk to doing it yourself though, be careful where buy the screen. A friend of mine bought a replacement screen for his N95 but when it arrived, it didn't work. As it had been shipped from Hong Kong it wasn't worth paying return shipping so he basically wasted his money and his N95 never got repaired as he wasn't willing to risk it again. Had he just paid to get it repaired in the UK, it would have cost more but at least it would be working.