I had to start talking it up 10 months ago, continually remind my management to set aside $$ in the budget, and swear on a stack of technical manuals that it wouldn't interfere with my test schedule, but, I'm actually going to NIWeek!! I need a HappyDance emoticon.
Not being aware of the info-labview discussion-heres my two penneth.
In general terms. An abstraction layer is an interface presented to higher level funtions, that hides the implementation. In terms of Labview, a good example of a HAL (hardware abstraction layer) would be VISA.