Actually, we don't even know if he is a legendary or not. There are plenty of arguments for both he is or he is not legendary, but there is no proof leading either way. Sure the demo screenshots of Diamond and Pearl might suggest he isn't, but it's just a demo.
Btw, Lucario FTW
The evidence for can be summed up in a single line: He was in a movie.
Everything else can be explained.
-Caught in a Masterball? Take a good look at the move Anticipate... and imagine a developer trying to catch ANY wild pokemon that spammed it.
-Wave Guiding title? That's nothing compared to Arcanine's Legendary Pokemon title.
-Signature move? Softboiled, jump kick, Megahorn... need I go on?
Compared to some simple facts.
He has a gender, without any advertised counterpart. (try and find official art of the Lati@s pair NOT together)
He has been shown at lower levels (30, at the lowest) than ANY non-event pokemon. (mew, Deoxys, MANFI...)
He was under consideration for having a preevo, according to the official website.
In the Pokejungle demo, he was wild.
In art, Lucario is usually depicted next to buoysel, who is NOT a legendary.
http://img45.imageshack.us/img45/3310/part1ou2.png
http://img145.imageshack.us/img145/3658/part2mo4.png
(note the absence of any known legendarys from that lineup)
His pokedex number was leaked, in the low 100s. Doripian had a higher pokedex number than it.
Care to make a bet on Lucario's legendary status?