SPLA is relevant if you host your solution for your clients or if you are looking to move to SaaS. It has popped up a few times in recent discussions and I was about to give a brief explanation here when I spotted a US colleague had already done a great job of it. I have copied the text here for simplicity.
"I was recently tasked to understand and explain the SPLA licensing model to an ISV, and thought that the experience would be worth sharing. Since folks love to read about licensing about as much as they love, say, root canals, I'll be as brief as I can - here goes:
For example, if you were considering hosting MOSS on a pair of 2-way dual core Windows Server 2003 R2 Standard servers with a 4-way dual core database server running SQL Server Standard on Windows Server 2003 R2 Standard behind, you would be charged for:
every month, and that's it - no CALs to worry about. You can even use the service yourself, but I think the only restriction there is that you cannot be the majority subscriber to your service in this case."