I have the same problem on my own Media Center PC. I don't know where the problem is but it may not be with Zap2it. After some investigation I found out the following. When Media Center connects to collect channel and listing data it connects to pes.tvdownload.microsoft.com. This seems to be a Microsoft website and that there does not seem to be any direct connection to Zap2it.com .
I would suspect that on or around the 17 August - 19 August that there was some change either at Microsoft or Zap2it that interrupted the flow of data available from that website. Until enough people start complaining about the problem I'm sure that they may not actually realize that the data is not being kept up to date.
I don't know if Zap2it also handles other countries but considering that another Media Center user in the UK seems to be having the same problem it may be Microsoft that broke the updates unless the UK listings are also provided by Zap2it.