10% down is pretty much the standard now. Fannie and Freddie started the lower LTV/CLTV back in December. Now, PMI is just following their lead. If the appraiser marks the box, that says a declining market, then the LTV must be reduced by 5%. Even if they do not mark that box, and the automated underwriting system could come up with a declining market area warning, or if the MSA (Orange County being one of them) is determined to be in a declining market by <a href="http://www.ofheo.gov/hpi_city.aspx">OFHEO</a>, or is in a <a href="http://www.pmi-us.com/media/pdf/products_services/eret/pmi_eret08v1s.pdf">high risk market determined by PMI</a>, then the LTV will be reduced by 5%. Do note, that OC's MSA has been ranked #1 for highest risk by PMI for quite some time.
Translation: You need 10% down to buy a home in OC, period. Unless the appraiser is secretly the next Jack Kerouac, then I doubt any lender, Fannie Mae, or PMI is going to believe it is not a declining market, when several other data sources say otherwise. If you need to pay PMI, then you better have 10% down.
On an anecdotal note, I visited a non-Irvine new home development, where I know the sales agent. A sales agent I worked with, who knows I can't be BS'd. He straight up said, Fannie Mae has determined the area as a category 5 (ironic that is the worst of hurricanes), and you need to have 10% down.