When two proteins form a specific complex with each other, how does that come into being ? First they need to bump into each other in some way, probably the wrong way round and very non-specifically, and then they will wriggle around somehow to find the right fit ...

NMR spectroscopists have developed some horribly complicated tools to describe that fleeting first meeting between two proteins that will form a stable complex later on. Read my story here:

http://www.rsc.org/chemistryworld/News/2006/October/16100601.asp

... and note that very similar phenomena can be occur between  people, though they are much easier to observe than proteins :)