It happens sometimes dust gets in during the actual assembly process and sometimes it get in during usage. Take my MT4G for example when I got it I thoroughly examined the display for dead pixels and dust underneath the touchscreen. I didn't have case or screen protector as I order them and took 1 week to get here. But within week I found one so you can see they will eventuality get in, nothing new or to worry about. Even if your neat freak like me chances are when screen is on you won't able to see it directly unless you tilt it in odd angle, but I know its there so just stop thinking and you will be fine.
Sent from my HTC Glacier via UMTS