Lots of people out there are asking the same questions regarding SharePoint licensing. Are CAL's for concurrent users? Do I need both Standard and Enterprise CAL's? Can I mix Standard and Enterprise CAL's in a given environment?
It has been difficult finding definitive responses to these questions, and it seems like if you ask 5 people you get 5 different opinions on the licensing. Finally I ran across this great FAQ from the Microsoft Office group specifically on SharePoint licensing, which answers the above questions and many more:
Microsoft Office SharePoint Server 2007 frequently asked questions - SharePoint Server - Microsoft Office Online