I am getting a invalid serial number when I try to upgrade my Unity 2.6 Free to 3.0 Free. But I don’t have a serial number. Nor do I need one based on the upgrade page. I filed a bug report. Is anyone else having this issue?
Same here, I did try manual and automatic activation, same result (didn’t try on the mac yet)
Yes, I downloaded the new free version, too. When I start the engine, a message pops up and asks me to Register. I choose ‘Internet activation’ and then choose Free. I do not enter any serial number, because I do not have any and I get the following message:
The serial number you entered is not valid. Please recheck the serial number and try again. To get a trial, contact support@unity3d.com.
I’m having the exact same issue.
same here …and it installed over the other unity 2.6.1 …now i can’t do anything…
Same here - I filled a ticket… guess they support guys will be busy for a while today/tonight 
I have the same problem
I had the same issue. I emailed support as no one mentioned doing that in any of their posts. I assume they check their email more than the forums. Hopefully they will respond to the issue shortly.
If your serial number does not begin with U3, you are using the wrong serial number and need to get one from Unity.
I have the same issue, and afaik I never got a serial with the free 2.6 version.
Free users do not have serial numbers. There was no purchase made. We are attempting to register our free versions which is supposed to be possible without ordering one from my understanding.
Me neither deps, but that may have changed with 3.0.
Its working for me, but I got my serial number in July beta.
I would imagine they are giving paying customers higher priority, which makes sense.
Yepp, it does. It would be ok to require a serial too, as long as the free version stays free. It just sucks that I apparently have to wait to get to play with it. But I will happily wait until more information is available. 
I would assume that the license handling server is just a very bit flooded by thousands to tens of thousands of requests + the last people rushing into the store to grab it at reduced price
Same problem here: can’t activate this version and the previous version was uninstalled. I guess that will teach me to download a new release of Unity as soon as I see it. But it was so hard to resist!! I hope this is fixed without having to redownload it.
This is probably just due to demand on the activation server, but we’re looking into it right now.
If the automatic internet activation fails for some you can also use the manual activation page to activate your copy:
https://store.unity3d.com/manual/
When you select Enter Serial number, pick Manual Activation. Then Save the ILR file and upload it to the page. When you entered everything it will automatically download an ILF file. You can load the ILF file to activate Unity using the Read License file button.page.
In some browsers the downloaded ILF file gets an xml extension. Please remove the xml extension in order to load it into Unity.
it doesnt work too
Authentication Error
The product activation system has denied activating the product. Please contact Unity support for further assistance.
Tried that and end up with
"Authentication Error The product activation system has denied activating the product. Please contact Unity support for further assistance."