Almost every day there's a popular blog post that misses a large portion of its audience because the author wasn't caching. You'd think that more authors would plan for a traffic spike from aggregators, since that's sort of the holy grail of blogging.
eh, I've lived through several spikes in the 6 years I've been blogging - the real holy grail is watching your search engine traffic increase bit by bit, month on month. I've been on digg's frontpage (back in 2007ish), linked to from xkcd's blog, I had a 20000 visit spike from reddit just this week. The spikes are fun, but they fade away after 2 or 3 days. The xkcd traffic lasted longer but that's because Randall didn't update his blog for weeks after the update that linked me.
But yes - even so, it's not nice to have your site die in the middle of a huge spike. It's just possible that that's the day Larry Page would have decided to tweet about you.
Generally using just mysql_connect is faster than using mysql_pconnect. On the server I admin for a large website with millions of hits a day we don't use mysql_pconnect.