A WebmasterWorld thread discusses a topic we discussed a while back named AdWords Adds Account Size Limitation. Basically, at some point, Google does not allow you to add more keywords to your account, they have some account size limitation.
Some forum members say that to increase that keyword cap, all you need to do is call your Google representative and beg. They can increase it for you.
AdWordsAdvisor replied to this thread, in response to what that cap is defaulted at. Some believe it is 50,000 keywords or so. Here is the response;
If you run into a keyword limit and need more, you may certainly contact AdWords support with a request. They'll review your account, and reply dependent on what they find there:
* An account chock full of thousands of keywords that are carefully selected and performing well? Your request will likely be approved.
* An account chock full of thousands of randomly picked keywords that aren't targeted to the products or services advertised? You'll likely get suggestions about how to streamline your keyword lists, and more effectively use keywords within the current limit.
So give your rep a call.
Forum discussion at WebmasterWorld.