why should someone glue them together???
ok - the dust will stay outside if it's sealed - but thats it!
it's a simple calculation
let's say - the digitizer costs 15$ and the lcd costs 30$
that could be the same price as the whole unit - BUT:
if you drop it again you will most likely kill only the digitizer.
that means you will only need the digitizer if they are not glued together.......
You cannot glue them together. Whoever sold you the LCD separate from the glass either did no know what they were talking about or lied to you.
You must buy the display assembly as one piece. You can buy the proper part here:
I just thought I would amend this answer based on some recent research. Here in Boston I met a technician at Warlox Wireless who does in fact purchase his replacement LCDs in two pieces. Apple uses a liquid/solid adhesive mix to attached the glass to the digitizer.
It is very possible to do this yourself, just not very easy.
Replacement display assembly includes LCD, front panel glass, camera bezel, and speaker mesh. Replace a broken, cracked, or shattered iPhone 4 display with this assembly. Compatible with iPhone 4 GSM/AT&T models. Black and white options. Kit and part only — 22 available at 64.95 each.