I added a quick/dirty high score table to a project of mine a while back and not being at all versed in internet security/etc it turned out sort of flawed. Surprisingly, with nearly 1,200,000 recorded scores, I've only had maybe 5 or 6 occasions of batches of blatantly incorrect scores reaching the top of the board. Most of the scores even looked more like a glitch in the game, than a true "hacking".
So I guess an important point is: make sure your game's scoring system is airtight, or at least do some really good score feasibility checking; now, this game I'm talking about was a Ludum Dare 48hr entry, so it wasn't the most stable thing around.. but on the whole I think it's often more likely that the casual player will discover/exploit an ingame glitch than having someone directly "hack" the leaderboard.
That said, I'm working on a rewrite of this project right now, and I'm going all out with the obfuscation. I won't go into too much detail, but I basically have all scores submit a key value based on a bunch of random values and hashing and a magic string, then any score that passes that check and is high enough to make the actual "Top X" leaderboard has to pass another round of validation (this time with an expiring key value generated on the server end, and more thorough feasibility checks).
I'd also advise using a packet tracer of some sort to test out what sort of things are visible (I was originally doing a much simpler verification that meant someone could use a packet tracer to find and duplicate an uploaded score's http request, without knowing the magic string or anything (meant you needed a legit score first, but you could send duplicates of that score as much as you wanted..)). I used Wireshark for testing this.
Huh, this turned out a bit long, but hopefully it helps...