I've always had to wait at the Genius bar, infact I've just come back from the Covent Garden store, where despite being early for my appointment, I had to wait nearly 40 minutes before they saw me.
I think the reason for this is that they have different levels of Genius' based on what your problem is. That's why they have somebody acting like a kind of triage and asking you what the problem is when you sign in. If it's a hardware problem with a Mac, such as I had, it seems like it takes much longer.
Stuff like iPhones and iPods doesn't require so much knowledge and is much quicker to diagnose and sort out. For instance in the time I was waiting, there were Genius' actually free who were on the iPhone/iPod bit.
I've always had to wait... but then I've never taken back an iPod or anything. Today was the longest yet though.