Depends on the restaurant. Most of the times the terminal doesn't support PINs and so it falls back to signatures. But some restaurants use PIN-capable terminals in which case that issue occurs.
I've had a restaurant in silicon valley (three years ago?) tell me that they couldn't use my card because the machine kept asking for some number and they kept entering zeroes and it didn't work.
That's only for US debit cards where a debit transaction requires a PIN code, but a credit transaction prefers signatures. For credit cards with PIN preference it shouldn't make a difference, as it's up to the card to decide which mode to use (based on the options that are supported by the terminal).
I know some terminals here in the UK lie about their capabilities, for example they'll tell the card they're offline only even if they do support online transactions.
