Chris_D, Search Engine Watch moderator reports the Introducion of "quality-based" minimum bids by Google AdWords. The detailed "what is" can be read at https://adwords.google.com/select/qbb.html.
Basically;
If you have any on hold keywords that you don't want to trigger ads, we suggest you delete them from your account. This is because any keywords with a high enough Quality Score and maximum CPC could be activated and accrue ad clicks.
The discussion on if this is good or bad for advertisers can be read at the thread.
