I have an iphone 3g, with a power support anti-glare screen protector and an incase slider case.
The power support screen protectors fit PERFECTLY.
I mean, seriously... perfectly. There is absolutely no gap around the screen, or in between the case and the screen line. However, the tight fit makes it very difficult to put on accurrately.
See if you put it on even slightly wrong, then slide the case on... it will cause bubbles on the screen protector because the case fits so snug.
Get it though, and take your time and make sure you do it right. Get an extra just in case you mess up the first one.
EDIT: Oh, and to do it right...... do the following:
1) Make sure you have the iphone with no case on it
2) Put only the bottom of the incase slider on
3) Clean the screen now with the bottom on (make sure it's completely dust free)
4) Apply the screen protector, so that it fits perfectly to the bottom of the incase slider... just up to it... and make sure the screen protector has the equal amount of room on each side of the chrome.
5) Put on the top part of the incase slider