I usually just tell people that I haven't set my prices for that far in advance yet, but based on my prices now, the ESTIMATE would be about $XXX. I have never really booked anything that far ahead...I get a lot of inquiries for people just planning their budget, but you would have to decide on what to do if they did want to book with the understanding that the prices may go up, or if you are going to decide to lock them into that estimate price as an early-bird special. I would be interested to see what other comments are!
I generally won't quote a price until they are within 9 mos. of their event. If they insist then I add 15% to my current charges and tell them the only way to secure that price is to pay a 50% or $300 (which ever is greater) non-refundable deposit. That way if my prices have to go up due to inflation of supplies, the rate I quoted generally is still substantially higher than that actual inflation rate, if they stay the same then I just consider the additional 15% a PITA charge, because most brides that price shop for their cakes a year or more in advance and are pushy enough to get me to quote them that early usually are or become bridezillas 
i tell them we can't intellectually talk pricing 2 years in advance, but they can comparison shop today's prices to see how everyone compares to each other and that should give her an idea of where she might want to go to book.
I'm also on a catering site and one caterer out west says he quotes 2009 at today's pricing, 2010 at today's pricing + 15%, and 2011 at today's pricing + 30%. I prefer to tell them "call me later".
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