I have been in a rather long dialogue session with an MSN AdCenter rep. about why my broad match type keywords weren't triggering my ads. At first they claimed they were seeing the ad triggered when they searched (both plural and singular). I tried from multiple IP's with no luck.
Finally, they admit that the broad match type WONT catch plurals, as their Help window claims that it would. Below is a summary of some of the emails.

On why plurals aren’t triggering my ads:

AdCenter claims they use something called 'normalization,'which finds and flags duplicate keywords in one's keyword list. Thus, MSN would treat "blue widget" and "blue widgets" as different words. If you want your ad triggered for both singular and plural, you will have to add these to your keyword list.

When I pointed out that the help pane claims that it will catch plurals and singulars:

The AdCenter rep explained to me that the information on their help pane (which I will list below) is outdated and asked for the web admins to update this information. So, basically the broad match isn't as broad as they claimed, or as many of you thought.

MSN AdCenter's definition of their broad match type:

Broad match triggers your ad when the word or words in your keyword appear, in any order, in a customer's search query. For example, your keyword "red flower" would match "red flowers," "flower is red," "flower's red," and other variations, but not just "red" or "flower."

Just a heads up all..