I think you are pretty lucky because this is the first time I hear Google giving a warning instead of closing the account all together.
All they are saying is someone has been messing around with the ads on your website, and it's only fair to the advertisers they don't charge for those clicks.
Google explained before that they won't go into how the invalid clicks were made because people wanting to cheat could take advantage of that. I know it can be frustrating but be happy they're not blaming you.
They didn't ban me (yet) but they do say - "In the future, we may adjust your
payment for any days during which invalid clicks occurred "
Suddenly my revenue last few days ( for same amount of clicks) is HALF of what it used to be, so I suspect they alreday are "punishing" me....
How can we be sure Google is not abusing its own system to divert revenues to itself on the expense of publishers who dont even know what they did wrong?
From what I remember Google has already been hit with Click Fraud themselves... don't be surprised when they ban you. Best is to look into the future. Maybe YPN...
They will ban you, take your money and not refund the advertiser. Don't rely on Google to feed you. Maybe create a script for yourself and your company/websites. Want more info just ask and I will explain in detail
You are a luckiest guy i have ever heard who got warned by Google about invalid clicks ! Normally they shoot first and ask questions later [they don't ask questions at all most of the time].if your payments cut for again i suggest you to remove Google ads for a while. and ask their help to see wuts going on. Since they are already nice to you they might help you farther.
In any case you are very lucky to have yr account active still. they wont accept you again for any reason if you get baned for invalid clicks. What matters most is keeping the account. So check your logs , email them and cooperate with them and get it sort out. good luck.
I thought that too. MSN has baged both Digg.com and Facebook.com others might move too. Also if MSN and YPN start accepting international publishers Google might have have to face 2 serious competitors.
Save your money 
