What does being a centaur get you? Well, for one, pretty great ability scores. +8 strength is good on its own, and combined with +4 dexterity, +4 constitution and +2 wisdom it gets even better. The minor intelligence penalty can be safely ignored.
A centaur's four racial HD are of the monstrous humanoid type, meaning they get d8 HD, full BAB, good reflex and will saves, darkvision, a small amount of skill points, and proficiency with simple weapons (and in this specific case, also the composite longbow and longsword). That's... not too great, but I guess it could be worse.
Centaurs are large, but don't get reach. They get a small amount of natural armor, two natural hoof attacks and the above-average speed of 50 ft, as fast as a heavy horse. Finally, they deal double damage when charging with a lance, which is probably the best strategy available to them from the get-go (well, as soon as you get proficient with a lance). One may even try to somehow get a centaur to ride a mount and deal triple lance damage.
So what LA is fit here? Fact is that centaurs give some very good things to melee characters. Add to this that even with +1 LA, 9th-level maneuvers remain available, and I'm having trouble not rating them at least that high. The permanent doubling of all lance-based charge damage isn't something to scoff at either.
+1 LA for now.
EDIT: Also, the two-month anniversary of this thread! Woo-hoo!
EDIT2: Turns out you can take Spirited Charge and apply it to your attacks, according to RoF. That's good. That's really good.