From Belinda, host of kid radio show Greasy Kid Stuff:
When we have playdates with new kids, or kids we haven't seen for awhile, I ask the parent to email me a picture of their child. This helps my kidlet get past that awkward first meeting stage. She already knows what the mystery playdate child looks like and we've talked about her, used her name, and pondered what they will play together. It's worked pretty well!
In fact, this hack worked on a playdate between Belinda's and my daughters!