Looked into this a bit further. In
https://github.com/bitcoin/bitcoin/blob/master/src/chainparams.cpp there are 6 hardcoded seeds for the main network:
vSeeds.push_back(CDNSSeedData("bitcoin.sipa.be", "seed.bitcoin.sipa.be"));
vSeeds.push_back(CDNSSeedData("bluematt.me", "dnsseed.bluematt.me"));
vSeeds.push_back(CDNSSeedData("dashjr.org", "dnsseed.bitcoin.dashjr.org"));
vSeeds.push_back(CDNSSeedData("bitcoinstats.com", "seed.bitcoinstats.com"));
vSeeds.push_back(CDNSSeedData("bitnodes.io", "seed.bitnodes.io"));
vSeeds.push_back(CDNSSeedData("xf2.org", "bitseed.xf2.org"));
Whereas testnet only has 2 seeds:
vSeeds.push_back(CDNSSeedData("bitcoin.petertodd.org", "testnet-seed.bitcoin.petertodd.org"));
vSeeds.push_back(CDNSSeedData("bluematt.me", "testnet-seed.bluematt.me"));
Both of which appear to be down as of May 20, 2014.
Still, I don't understand why this would bring the testnet client down. Doesn't it keep a list of previously connected peers?