Before image by kind permission from Tracy and Matt's blog
Step 1: Take Apart Phone
Remove the battery, and sim, and then work a small plastic shim (I use guitar plectrums) around the join between the two halves.
There are two clips on the top of the unit that keep the two haves together.
**When you're doing this, remember there is a ribbon cable running from the camera/speaker unit on the back to the motherboard. Be careful, and just lift the connector up and away from the motherboard as you lift the back. Be careful to not lose the power, camera, and up/down buttons which are now loose.
Once you've done that, inspect the motherboard. Here's a closeup. In the red circle is the power button, and a screw hole. Now, that screwhole wasn't one you've just removed a screw from...
If you have a screw in this hole, then my fix does not apply to you. Replace that ribbon cable by gently pushing the connector back, ensure your buttons are back, and put your phone back together.
Step 2: Ok, So You're Missing This Screw.
The picture below is what you get if you gently remove the red earphone cable from the motherboard, and remove it. This is the top left corner of the unit as you look at it from the front.
When I was getting dust, it was in a diagonal line from there to the center of the screen.
Notice how there is a rubber seal around the outside of the screen.
*EDIT* Doh, I've just noticed that photo of the inside of the front cover, that circle is actually on the wrong side of the seal, that's the front camera hold there. As you look from the front of the device, that's the top right corner. The foam seal pictured runs around the seal so that photo is still ok to show you what I mean, but remember the hole is actually on the other side of that.
The next photo is a closeup of the front of the removed motherboard. That is the hole from the first step. Note how it's inside the seal. In fact, when you close the phone back up, that hole will is half covered, half not by the seal, and that will compromise the seal, and allow dust into the device.
Step 3: So, What's the Fix?
I considered hot glue for a while, but that looked like a nightmare. So in the end, I used Blu-tack!
I hope people outside Europe know what I mean. That putty like stuff you use to put up posters and Christmas cards with.
Just roll a small amount into a tube shape, and work it into the hole on the motherboard from the front. Then shape the blob so it conforms with the lines of the unit. You'll want to build up the blob so it covers the hole, and fills the gap between the two lines of the metal around the edge of the screen.
Don't use too much, and don't put it anywhere else.
Step 4: Clean and Close!
Keypad in the hole, motherboard back in, refit that red and black cable,
Carefully turn the phone over, and make sure the screen is clean inside (and that you can't see any blu-tack)
Refit your buttons (check they click before you close it), bring the rear of the unit back, and replace that ribbon cable (it sits in a small square socket and just pushes in) and then replace the back of the device.
Six screws and two rubber bungs later, you should not have any spare parts left over. ;)
That should be the last you see of dust getting into your i600.