This is actually somewhat of a touchy subject.
On the one hand, many DMs and Players say that they value the "natural" growth of the PC and the impact that PC has on the campaign and vice-versa. We like to think that Players and DMs both are going to avoid using external/real-life knowledge as a "crutch" or undue influence on the decisions their characters make.
On the other hand, this is a game. As a game, there are strategies and tactics that both Players and DMs use to confound, confuse, prepare, and -- yes, it's true -- win.
I do not think that a player should be penalized for creating a character at first level with the goal of achieving a specific PrC at level 9 so long as the player knows the PrC is available in the game world. Afterall, many of us had desires and goals to achieve some kind of profession or notoriety in some field or career at one point in our lives well before we had any clue as to what it took to achieve said goal (and many of us continue to have ideas and attitudes about certain desires even after we've figured out what it takes to achieve these goals). I've never had any problem with players working to achieve a specific alignment, PrC, or what-have-you so long as they are doing so within the dual-framework of role-playing and gaming.
That in mind, I think that there are clear signs when the threat of meta-gaming overwhelms the desire or goal to role-play. If at every encounter, you have a player who comments on the attacking monster and his character conveniently knows that it's a Pit Fiend despite the fact that the character's a 10th level country-bumpkin who's never had a discussion with the local cleric about the afterlife and the various worlds beyond, then you have a problem. A sure-fire way to handle this is to introduce monsters in a different fashion. Increase the HD, swap out spells, or grant levels; this may require that the monster is introduced later than you'd prefer, but at least it will force the meta-gamer to always second-guess himself.
Beware as a DM for doing the same things you accuse meta-gaming players of doing. For example, if you know that your players like to fly, like to haste, and are always immunue to fire, every single enemy they fight shouldn't be prepared to counter these elements. Of course, a recurring adversary who's confronted the PCs before may be prepared, but new enemies won't necessarily be prepared. Likewise, don't ensure that adversaries are immune to newfound or historically successful tactics/powers. That stinks of foul-play and spite and gives the appearance that the game's about DM vs. PC and that's not the point.
This is a game. Never forget that. But, it's also partly live theatre. The two can meld well and there needs to be that understanding from the get-go. Discuss with your players your expectations and be certain that they will offer their positions as well.
Good luck!
