So, what if a major brand name forgets to renew it's domain? If I spot it and then register it - is it mine?
What Gosse said.
As far as "is it mine?" depends: most of the time, yes. Since the owner obviously didn't use it actively enough to either not notice the 45+ day downtime or not care enough to renew it. However, if the domain name itself either is or contains a trademark, you can still run into problems registering it because the trademark owner will have a very good case contesting your ownership as a trademark violation, whether it was up for grabs or not.