From the MSN Blog:

“Even though these human-edited descriptions provide a lot of value, with human editing may come human error, bias, descriptions getting outdated, or the editor’s text may simply not suit the webmasters who want to be represented in their own way. What has bothered the webmasters previously is that when search engines preferred search result descriptions from dmoz.org, they did not empower webmasters to opt-out of those descriptions. This can be especially annoying if the descriptions from dmoz.org are outdated, or just plain inaccurate”.

Indeed.

So, all you need to do to avoid this problem is add a meta tag:

META NAME=”msnbot” CONTENT=”NOODP”

I had this same problem – Google was using the DMOZ description for one of my sites. The way round it was to use a meta description tag. The DMOZ tag Google were using was wildly inaccurate, which really affected click-thru rates.