It's not the norm, but it has helped us a bit, and to be honest, it's still less expensive than most recruiters. When you're hiring positions between $90-$150K, recruiters want 20-30% of that first year's salary, so spending $24K to build a more organic pipeline makes a lot of sense.
I'm not sure if it's the norm but I know SEOmoz are desperate for top talent and are willing to pay for it. Their web-index is pretty complex and so the engineers they hire need to be highly skilled.
Here are our 8 open positions currently - http://www.seomoz.org/blog/8-open-positions-at-seomoz and please note that for software engineers, we pay a $12,000 referral bonus to anyone who refers an engineer we hire as well as $12,000 as a signing bonus to the engineer. Details here - http://www.seomoz.org/refer-an-engineer