This past week Microsoft announced a change in their support policy. The revisions were made on the support site and also announced on the CSS blog. These changes say that if your particular service pack as reached the end of its support lifecycle, you will still get some support from CSS. You might be asked to upgrade service packs for additional help, but you won't be just ignored as some people have feared.
In the past, when support for a product or SP ended, customers had the option of purchasing an extended support agreement, which would ensure resources if you found a bug of some sort. I don't know many people that purchased these for Service Packs, but I do know some customers got them to ensure a product would be supported. Without those agreements, I have heard that the support group from Microsoft would refuse to provide support until you had upgraded service packs.
That hasn't been my experience with SQL Server. I have had to call PSS, now CSS, for products that were EOL, and the technicians that I spoke to made an effort to help me. They definitely said that patches couldn't be issued, but they spent time on the phone with me, working through issues, looking through logs, and trying to determine what could be done. I've been very pleased with the effort on the part of MS support over the years, something that I can't always say for other vendors.
To me, this change isn't much of a change at all, but I still think it's a good move to codify the policy and let customers know that they aren't going to be forced onto a treadmill of constant upgrades. SQL Server 2008 RTM just ended its support lifetime, and this should give you confidence that you can still safely run this version of SQL Server in your production environments.
Not that I'd recommend it. I'm not sold that applying CUs without a reason is a good idea, but I do think that keeping current on Service Packs, at least within a year of their release, is the way to go.