I won't know anyone
This situation will persist until you go to a conference and meet some people! Then, you'll know some people at the next event. When I went to IPC in Frankfurt in 2007, I knew nobody but while I was there I met Derick, Sebastian and Zoe ... and these three are now conference friends wherever I go! Lots of people at conferences are there on their own and will be happy to chat to you and find out who you are.
Its too expensive
While I'm lucky enough to have the support of my employers, Ibuildings, to send me to at least a couple of conferences per year, I've worked for plenty of other organisations that didn't invest in their people. I think this is unforgivable but the reality is there are plenty of us in this situation. If you are paying your own way to these events I can appreciate that $1100 (~700 GBP) plus international flights plus a week in a hotel in Silicon Valley can seem pretty expensive if you want to get to ZendCon. But there are cheaper and closer conferences are available for most of us - so do your homework and get to something you can afford, even if you don't do it every year. I've yet to get to a conference where the cost outweighed the benefit so in my view this excuse is invalid.
My employer won't pay
No, well, see previous point. This is true for plenty of people and while I don't have any numbers on people paying their own way - they do exist and they almost invariably move on to work for employers that do invest in their future. Do this for you, not for them.
I might have to talk to people/strangers
This is the excuse I hear the most often, or a variation on this. Actually you don't have to talk to anyone if you don't want to. I went to an event last year and introduced myself to a guy who said "Hi, I'm James. I don't have any social skills" and proceeded to say nothing further (his name may or may not have been James, I can't really remember). To be honest I didn't really think anything of it. Conferences are firstly about the technical content so if you want to come and get the technical sessions and then disappear again - that is your call. I can't agree this is a good idea but there is absolutely no pressure to be the life and soul of the party, and in fact if you want to sit in the corner and mutter to yourself that is also fine ... we're all geeks after all! Nobody will judge you, in fact if you don't talk to anyone probably nobody will notice you - just COME and you might be surprised :)
I haven't been to a conference
Why not? Pick an event you like the sound of, join in the preconference hype (more about this in my post about making friends at a conference) and see how it goes. If you hate it, then don't go to another. But don't stay home immobilised by lack of experience, you're missing out :)
Getting to a conference costs time, money and effort and if you don't want to invest any of those things in your professional development then I respect your decision. However if you think you'd like to attend something, but you don't know what to expect or you have concerns about what is expected of you, then try to put those fears aside and dive in! I think I've covered the things I hear most often - what excuses do you hear from conference-avoiders?