That's a good question that I've never given a great deal of consideration to although my first thought would be to try breaking down the issues.
Hiring a new developer usually boils down to one of two different sources:
Gain a lead from a current employee
In a lot of ways leads from current employees are your best bet. (Note I'm assuming you are only really looking at the leads from your quality employees.) A good developer will most likely never really recommend someone that they don't want to work with. I personally (although, granted this may be unique) have never really run into a situation where a friend was recommended that wasn't a good fit both temperament and ability. That being said no matter the source of the recommendation you will still need to go through a full interview process to ensure that the individual will in fact be a good fit.
Recruit a new candidate
If internally supplied leads can't meet your needs then your next option is to go recruiting. Probably one thing to consider is that good developers (assuming a fairly normal job market in their region) will generally be gainfully employed. By this I mean they are fairly satisfied in their job and employer. However a certain percentage of them will be open to new opportunities. How they go about being open to new opportunities varies among individuals but two things seem to be fairly common.
- Post resume/experience/etc. on a listing like Monster.com
- Occasionally peruse classifieds (which ones are fairly locale specific).
The developer's I know that have changed jobs have typically either accepted a position with a company that they applied for directly or been a lead developed by a recruiter. If the applied directly the it comes down to selling your company in a (guessing here) 200 word ad to get them to apply. Recruiter's are difficult since many of them will shove any warm body at a position in the hopes of making some money. That being said it doesn't mean that they can't handle the leg work of find those individuals posting on a job board for you and forwarding their resumes to you.
Honestly I'm not sure this is really much of an answer to your question. I think I can state fairly strongly that it's hard to find good employees (whatever your company does) and it's hard to recruit new good ones. Based on that and given how much of this is up to the each individual I'm not sure there is a good answer.