One of the bits that is often over looked in SOA is the importance of the consumer to an overall SLA for a service (after all the SLA is there to give the consumer assurance). I've been looking for a good example of what I mean and today I found it. I have a Canon EOS 350D, the old entry level digital camera, which is perfect for what I need. I've got 3 different brands of memory cards, all of which publish their performance SLAs. The two that interest me here though are the SanDisk Ultra II and SanDisk Extreme III. The later claims a minimum write speed twice that of the former. However if you lob it into my camera, set it on manual with a 1/4000 shutter speed and then hold down the button for 1 minute you get exactly the same number of shots out. This indicates that the issue isn't with the speed of the memory cards to determine overall performance but the speed of the camera in writing to the memory.
So here we have two services which have identical (one could say "uniform" :) ) interfaces but offer two different SLAs to consumers, these SLAs however place requirements on their consumers in order to be able to deliver the SLAs as advertised.
The point here is that a service cannot offer an SLA that reaches outside of its bounds without also placing direct requirements onto its consumers. Thus a true SLA is not simply something a service describes but is something that is negotiated between the producer and consumer for a given interaction.
Published SLAs are just the tip of the iceberg when it comes to delivering truly effective business SLAs that can be relied upon.