To make it work, you need developers who are true team players and recognize the future value of a good programmer. If your developers hate being bothered and think passing on knowledge to the younger generation, or think of it as pain, then you're pretty much already in a bad situation.

I've found that in these situations, the developers are always under the gun and suffering from a lack of time. I've also often found that they suffer from a dysfunctinal development process and several other self-defeating set-ups which suck up all of their time.

The time to teach other people anything isn't losing productivity since it should make both sides better. If you're just counting lines of code or time at the keyboard, you probably don't realize how much you're actually losing by creating information silos. If you can't come up with a way to effectively integrate a new person into your group, you'll be hurting when you lose key people (like I once did when he drove his sports car into a tree).

Every situation is different, so it's difficult to perscribe anything that works everywhere. Good developer docs (which means, not just the APIs, but how to use them), good task tracking, can help. Start new people off with small tasks, maybe related to wish list items or tool smithing, and gradually bring them up to speed on the whole project. Let them sit in meetings, etc. If you want a good programmer, you probably also want to keep them for a long time. A transition time of even several months sure pays off more than several short term bad programmers over the same time period. I've seen both situations.

The communication problem really isn't that bad. Give every new hire a sponsor: someone who you trust to be a good programmer (not necessarily the best), who can take the newbie under his wing for a while. Of course, this sponsor also has to have some skills at managing that, which a lot of programmers lack. Programmers, as employees, need to be more than just code slingers. A "senior programmer" who just slings code isn't really worth the title.