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:

• SPLA stands for Service Provider License Agreement
• SPLA allows ISVs to offer our products on a per CPU per month basis
• In this case, CPU means "socket into which a chip is inserted" - so multi core and/or hyper threaded CPUs only count as 1 CPU for this model
• Prices are determined by the price list which comes as part of the SPLA agreement

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:

• 4 MOSS (2 servers x 2 sockets/server)
• 4 SQL Standard (1 server x 4 sockets)
• 8 Windows Server 2003 R2 Standard (1 server x 4 sockets + 2 servers x 2 sockets/server)

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."