Firstly some people have been getting an "Invalid Serial Number" message while trying to connect online. While no-one is sure how widespread the problem is, it has been mirrored already in this EA Forums Thread quite a lot now. Apoc posted the following message:
I will tell you guys first hand here, that the same problem occurred with the release of Spore. Believe me, I was in those official Spore community forums a couple of months ago, ranting my head off as well. The problem is that authentication servers often cannot handle the load, and it will quite probably be a few days before EA can iron it out. It's just a waiting game. Your Serial number is 99.9% unlikely to be invalid, so don't panic.
The second problem is more of a printing issue. Some of the numbers that have been published are incomplete. So intead of having a full 20 digit code, you will have a 19 digit code with the end Digit left off. So you will either have to guess that missing digit, or phone tech support. But in my opinion it's best that you guess, and then write it in for future reference.
In both cases this is becoming more of a common issue with EA games. Spore had both problems respectively.. True, all these problems should have been ironed out after the issues with Spore's release. But all I can really say as advice is wait and guess.
UPDATE: Apoc has been busy, he has issued this update:
Too many multiple threads for me to reply in, so here is the official word on BOTH serial key issues right now.
SERIAL KEY ISSUE #1 - My key says it is invalid and I cannot get online
There were a small amount of keys in the early distribution which were not pre-loaded in to our backend, thus that is why it's tell you it is invalid. This is a similar issue we experienced during the Beta Test. This will be resolved by the end of day TODAY. By tonight, or at latest tomorrow morning, anyone experiencing this particular issue should be able to get online without a problem. Many of you may not even experience this.
SERIAL KEY ISSUE #2 - My manual has 19 digits instead of 20 for the serial key
To our misfortune, another small set of the first run of manuals misprinted the serial keys, leaving one digit off. Our Customer Support team will be posting a simple FAQ proces at help.commandandconquer.com where you can contact them and they will quickly help you fix your key.
However, there is a quick workaround if you do not want to go through that process.
The last digit of your serial key will either be a number from 0 - 9 or a letter from A - Z. Using a quick proces of elimination, simply type a number or letter at the end of the serial key until it activates. There is a maximum of 36 possible entries, so it's likely you'll hit the correct number or letter before then. It's not the most glamorous workaround, but its quick and painless and will save you some time from contacting Customer Support if you don't wish to do that.
We apologize for these two serial key issues. Both were absolutely out of our hands and while seemingly unacceptable, we hope you can understand. For most of you, this should not be an issue by tomorrow, especially when everyone starts getting online. Any more updates on critical issues or any of the above, I'll keep you posted.
Well at least the online issue has been addressed quickly. As for the misprinted key on the manual... at least it's a nice Real Time Strategy mini-game eh? OK, maybe not, but at least it's not going to cost you a call to Tech Support.