1st gen: fused
2nd gen (iPhone 3G): separate
3rd Gen (3GS): Separate.
4th Gen (iPhone 4): fused.
The feat you describe is nearly impossible. Trust me I have tried it and have repaired hundreds of iPhones of every model.
also for reference iPod Touch 1-3gen are separate, and 4th gen is fused. Fused gives the LCD and glass more strength and eliminates the possible dust, BUT adds cost. I think in the end fused is worth it.