IPhone – The Safari app has disappeared from the iPhone

Are you absolutely certain it's not there? There are a few reasons why you might not be seeing it, only one of which is due to the icon actually not being visible.

1. The obvious one, you're simply overlooking it in an obvious place. Unlikely I know.

2. The almost as obvious one, it's in a folder and you haven't looked inside the folder. Also unlikely, but less so.

3. There are so many apps installed that it doesn't fit on the 11 Springboard pages. This is the most likely, because you can have as many apps installed as you want (flash memory permitting) and they'll always be available via Spotlight search, but you can still only display 180 icons. If you use folders then each folder can contain 12 (or 16?) icons, but if you've hit that 180 icon maximum (11 pages x 16 icons + 4 icons in the permanent bottom row) then additional apps simply won't appear.

If the problem is that there are too many apps, put some in folders (or delete some apps) and then sync with iTunes and the rest will reappear. Alternately you can use the iTunes app organizer to rearrange things; in that organizer you can see all of the pages of apps past page 11, which are kind of grayed-out so you know they won't actually appear on the device.

Even if you don't have 11 pages of apps, try the iTunes app organizer anyway and see if you can find the app icon then, and try moving it to a more conspicuous place.

Have you checked that when you re assembled the phone you lined up the positioning of the dock connector correctly, a tiny bit off and it will cause the home button to not work.

See this solution for removing the back of an iPhone 3GS for disassembly instructions. On step 29 you will see the importance of this components positioning.