I'm a Linux SysAdmin in Sunnyvale, clients are all over. I don't do a lot of consulting, maybe one day a month or so, give or take. But I usually charge around $100/hr. I've gone as high as $200 and as low as $80 in recent times, but $100 is usually the target. Of course, my prices float based on how much I want the work; If prgmr.com just handed out a big credit and/or I'm drooling over some new hardware, you can often get a deal; when the cashflow on my VPSs is good, on the other hand, not so much. Last time I did full-time contracting through a body shop I got $72/hr, but this was several years back, and I've published a book since then.
I'm looking to have my company do more consulting as a way to raise short-term revenue, and as a way provide my employee(s) a way to do higher value work (and thus earn more money) without leaving my company for more profitable employers.
Ideally, I'd personally do about one day a week of contracting like this, and as employees become more experienced, slowly transition them from support to consulting.
As far as I can tell, rates depend as much on the client as on your skill level. I've had some clients laugh at my rate and offer /more/ - while others do the opposite.
I'm considering retainer type deals, and I'd be interested to hear how retainers have worked for other people.
edit: attempted a less-objectifying reword.