I keep seeing this brought up and I honestly am not sure where it started. Ok, that's not true, I have a pretty good idea where it started.

Yes, with 490 Defense, you are unable to be crit by nearly all mobs. Unless fighting mobs or players with higher than 5% crit.

To quote Satrina's Evil Empire Guide:
Which of those is it? We just don't know. In any case, the phrase "crit immune" is an extremely bad choice; don't use it. We can look at the Malicious Instructor in Shadow Labyrinth that is known to land crits on 525 defense (7% crit reduction in theory.) Quite possible that this mob has an increased basic crit chance since it isn't even level 73 and consistently crits 490+ defense. Also possible that their "Mark of Malice" debuff either increases their crit chance, or reduces your crit reduction - though there is no visible change in your defense skill while you have the debuff (like the Twin Emperors' Unbalancing Strike does.) Either way, this mob consistently crits "uncrittable" defense skill warriors, but we can only speculate whether the Instructor has more than a 7% crit chance so that it beats 525 defense (soft cap), or maybe 5.75% to just beat a hard cap that you reached at 490.

So, once in a blue moon, or just higher crit chance? This is the realm of opinion and holy war amongst warriors. Personally, my money is on a higher crit chance given the far greater number of mobs that do not land crits on 490 and higher defense, rather than an exception case in the combat mechanics. This is, of course, my opinion and not a known fact. Either way, the number of crits you will take at 490+ defense is really not worth worrying about.

Why am I writing this? More as a service to Satrina, whose had to ward this question off repeatedly on the Warrior forums. Hopefully just one person who reads this will beat Satrina to the punch and save one person from carpal tunnel.