Nintendo did deliberately brick Wiis, though it wasn't for usage of the Homebrew Channel. Rather, it was for having a region-changed Korean Wii. This manifested in the form of Error 003. Essentially, starting with version 4.2, the Wii would attempt to use the Korean Key to encrypt a specific series of bytes, and then compare the result to hard coded values. If the result matched the hard coded values, it meant that the Wii had the Korean Key, and thus had been tampered with -- making it throw Error 003. If the result did not match, the Wii continued on its merry way.
Source:
http://wiibrew.org/wiki/Error_003
I'm guessing Nintendo did this because there were people region changing Korean Wiis (which were cheaper), and selling them for a profit in other countries.