One of the more common complaints about the iPhone is that occassionally dust can get under the touch screen. This is a very irritating issue when it occurs because it is very difficult to remove the dust short of taking apart the phone , which we will get to in a while. These dust particles are often easily visible when the phone is off and quite often even when it is on.
Where does it come from?
So how can you prevent dust from getting under your iPhone's glass screen? The most common way that I have found dust getting under the iPhone screen is when it is able to get around the gasket surrounding the screen. There is a dust gasket which is essentially a seal between the iPhone screen and the body/backing of the phone. On a 3G or 3GS, this gasket can be seen in the little channel on the front of the phone, between the black screen and chrome case/bezel. In that little channel there is a soft gasket running all the way around the phone. The purpose of this gasket is to prevent dust from finding it's way under your screen.
Never stick anything into this channel and press on this gasket. Even applying a small amount of pressure with a finger nail or anything can open a pathway between the outside atmosphere and the gap under the screen. I actually personally did this accidentally when cleaning and soon after found dust trapped between the touch screen and LCD.
It's also possible for dust to enter under the iPhone screen via the charging port or SIM card slot although these are less likely.
How to remove dust that is under the iPhone screen
If you are unfortunate enough to get dust under the iPhone screen, there are ways to fix it. First off, if your iPhone is under warranty (length of warranty depends on whether or not you have AppleCare), take it to your local Apple store. When under warranty, most people have reported that Apple will repair the phone right then and there. If necessary, they might swap it out with another refurbished unit. Even if your iPhone is out of warranty I'd still recommend checking in at the Apple store first.
If Apple will not fix it, you have three options.
1. Hirred help- Hire a local cell phone repair shop to fix it. There are also national iPhone/iPod specialty businesses that you can ship your device to who will repair it. In some cities there are people who will make house calls and come to you and fix the iPhone. Places I called and found online were charging between $40 and $75 USD to clean out the dust. They guaranteed they would clean out the device and the iPhone would work, but can't guarantee it won't come back.
2. Fix it yourself- You can follow online instructions/videos on how to disassemble the iPhone and clean off/replace the screen. It's not nearly as hard as you might think, you just need to be careful. This will also void your warranty but if you are doing this yourself you probably are already out of warranty.
Note that with either of the above options, if there is a breach in the gasket or other sealing issue dust could eventually find itself back under the screen.
3. Deal with it- If it's not that bad and/or you don't have the money to fix it or the guts to attempt a fix yourself, just live with the dust under the screen. Eventually you'll buy a new phone (probably iPhone) and the problem will be solved.