i had this problem, wrote on here about it aswell..
i was setting up my connection and the update started, but then froze, luckily my dinner was ready so i just left it going.
after i had finished eating, maybe after an hour or so, it was still in the same place

so i tried to power the wii down with the remote, it wouldnt respond, i tried the reset, nothing, so i simply had to pull the plug, it was the last possibility.
after it had been frozen for such a while i guessed it wouldnt actually be writing anything to its storage memory, so pulling the plug wasnt so much a problem.
started it up again and started the upgrade again. well it went passed the previous point it had reached

.....
and THEN froze again

again i left it for a little while, just to see if it would actually get anywhere. it didnt..
i knew my net connection was fine, i know that wasnt the problem, so i realised it was most likely nintendos problem, server-side.
i just played some 360 and tried again a while later, it worked no problems
(also spoke to some others who said they had tried updating at similar times and said theirs had frozen aswell, backing up the idea it was nintendos problem...)
if you are waiting for an answer to this then im sorry this was such a long post, but you are probably gonna have to pull the plug