I found this blog post while researching real estate sites... seems like Microsoft has a major programming issue with their billing/credit system! http://www.kristafracke.com/blog/2007/03/15/microsoft-calls-and-asks-me-to-pay-for-their-mistake/
Thats very disturbing. I wouldnt pay them for it either... I just started a Adcenter account and Im already worried...