As Tom says in his blog, you can do RAC without Grid and you can do Grid without RAC. I am working on setting up 10g RAC and I am not necessarily planning on implementing Grid. So I am doing RAC without Grid. In my situation, I am only concerned about clustering multiple servers together to support the same database. That is what RAC does. RAC is used for Oracle database clustering. Similarly, I can cluster multiple Oracle Application Servers together to support my Web server needs. I do not need a database in this scenario, so I can have Grid without RAC.
The one big benefit that Grid lets me to is to put my Web server cluster and database server cluster in the same framework. I can scale both portions by adding more, faster servers into the mix. And with Grid, I can move servers from one portion to the other. All the servers are part of the same grid. So as Tom says, you can do Grid without RAC and RAC does not necessarily mean you are doing Grid. However, if you want to add a clustered database to your grid, you need RAC.