I have had all three nokia ruggedised(sp) phone starting with the 5210 then the 5100 and now the 5140.
The 5210 was the strongest of the lot, reasonably water resistant and shock proofed, IIRC it had "suspension" holding the electrics away from the case, but was a pretty basic phone.
The 5100 was the new improved version with a torch thermometer and a few other features, I dropped it from my pocket and broke the screen within a week! not as sturdy or waterproof.
The 5140 (5140i is basically the same with a better camera and software)had lots of new features including compass and add on GPS(which I haven't got), the keypad isn't even part of the face so there is a gap down the side that muck and water gets into.
Overall for pure ruggedness I would go for the 5210, the oldest one of the range!
They seem to have gone away from being functionally strong, to just having the look of strength. I tried out the 5210 by trying to inflate the case, to see if it was sealed and there was a little resistance, not for diving by any means, the newest phone has holes in the case so there is nothing to stop water getting in.
If you are after the latest blue tooth and gizmos go for any chocolate bar design and put it in a strong zip lock, you can still use it through the bag and apart from waterproofing it the bag will cushion it too.