I think the size of an emergency cash reserve depends on how much you have in liquid taxable investment accounts and contributions to your Roth IRA.
For instance, if you have at least a years worth of living expenses in taxable brokerage accounts, then you are in good shape compared to someone who has none. If the market crashes 50%, you still have 6 months worth. Market freezes (think Sep 11, 2011) and natural disasters (tsumani?) might delay getting to such funds, but they are rare events and not likely to keep you from your money more than a month.
It takes a few months before creditors will take drastic action. You can skip paying conventional phone bills (not prepaid), water, sewer, etc. a month or two. Rent is not something to be skipped, but there's probably more forgiveness in a mortgage. Property taxes on a mortgage-free property can be skipped for a couple years. Yeah, that damages your credit, but if the choice is damaged credit or bankruptcy, which is worse?
Paper savings bonds are an emergency fund, too, as Katrina victims can attest. Now that they're no longer available and you can only buy electronic records, I have slightly less faith in using savings bonds as an emergency fund... am I being old-fashioned?
So... I have 4+ years of living expenses available in taxable accounts (having about 3 years worth of unused margin!) and Roth IRAs. There's usually a month's worth of expenses in the checking account and at least a month's worth of expenses in the savings accounts. On top of that, I have a month or two worth of savings bonds. The taxable accounts currently generate 30% of our living expenses (that we're not drawing down).
In many ways, that all seems like overkill.