Reportedly, if you currently are already excluding your AdWords ad from showing on over 5,000 sites within the Google content network, you are safe and Google won't drop them from your list. So maybe it is now time to really start adding sites to your Site Exclusion. Here is how:
(1) Sign in to your AdWords account at https://adwords.google.com.
(2) Go to a campaign and click on the link as highlighted below:
(3) Make sure the Campaign Management tab is selected.
(3) In the shaded area beneath the tab, click Tools.
(4) Select Site and Category Exclusion.
(5) Choose the campaign whose exclusions you'd like to edit from the Campaign drop-down menu.
(6) Use the tabs to make exclusions. See detailed instructions for using each tab below.
(7) When you're done, click Save all exclusions.
(8) Repeat steps 5-7 for any other campaigns.
More details on using the Site Exclusion feature can be found at AdWords Help.
So be prepared, rumors of a 5,000 site cap is coming to Google AdWords site exclusion feature.
Forum discussion at WebmasterWorld.