To go with what Skallgrim and TwinBahamut said, I once read an article (I think it may have been on the pre-Dragon/Dungeon WotC site - I can't find it now) that described a scenario where the players played two or three parties of characters. Each player had two or three characters, and these characters were organized into "squads" or groups within a larger organization. They are often different levels, so a playing group might have one party of 10th level characters, one party of 4th level characters, and one party just starting out at 1st level. From session-to-session the group can decide which party they want to pull out for the given adventure.
The in-character explanation of this is that all the characters of all the parties are in the same organization, but just have different teams. The highest level group of PCs will probably be the leaders of this org, and they can send the lower-level groups on missions that are too easy for them (we don't want to waste our time on this) or that they can't do for some reason (we can't attend that party, the countess knows us and wants us dead).
The benefits of this setup are that players probably won't get as tired of playing the same race/class over a long period of time if their characters are significantly different. Also, if a PC dies you can "promote" a PC from lower levels into the higher-level party, if you want, which can help maintain group cohesion since the characters all know each other and you're not just pulling a random person off of the street and tossing him into a high-level party.
I don't think it's quite what you were looking for, but having each player create multiple PCs of differing levels may achieve your end goal (keeping players from getting bored with their race/class choices).
I really wish I could find that article because it had a lot of nice explanations of this playstyle. If anybody else knows where it is I'd appreciate the link.
