In the past we have heard about AdWords Account Size Limitation, where you are limited in the number of keywords your campaign can hold. We have even heard of remedies to this problem. But these, I believe, were all after the fact - after you have max'ed out your keyword list.
A Search Engine Watch Forums shows that one advertiser received an automated (looks automated) warning from Google that he soon will reach that limit. The alert said...
The keywords in your account are nearing an unmanageable size. We recommend that you reduce the number of keywords within your account. This will ensure that your account includes the most targeted and relevant keywords possible. Use our Find and Edit Keywords Tool to identify poor performing keywords within your account (such as keywords with few or zero impressions) and delete them.
I wonder if this is getting more wide scale.
Forum discussion at Search Engine Watch Forums.