gavinandresen/bitcoin-git — comment on commit 9db95d35d3c3bf1c41674c7e1e4da8816abb9690
File: src/base58.h line 296
By gavinandresen:

Because 11 is my favorite number.

... no, really, ask sipa, he has a scheme for bitcoin address numbering, and in that scheme testnet addresses are 111 xor main-net-number.