I was able to go to LISA in 07 and I finally get to go again this year, but only for the first two days and only for training. Some of the people I met at LISA actually had in their contracts that they go to the conference every year. The training I received was pretty good, but I learned the most from going to the talks and other related events. For example, learning how Goldman Sachs configures Nagios to monitor a 2,000 node HPC cluster and the lessons they learned from it. Or sitting in on a discussion about Puppet with the creator and the people that use it. It's hard to tell the guy that signs the check "I'm going to bring back this specific knowledge" when you don't know the details of what's going to be discussed vs. being able to show them a training outline.
How do you convince your employer that conferences also benefit the company (and get them to send you)?