Zapthink wrote up an article on when SOA is not a good idea.  This is their list of when SOA is not a good idea:

1. ...when you have a homogeneous IT environment
2. ...when true real-time performance is critical
3. ...when things don't change
4. ...when tight coupling is a pro, not a con

Of course the real message here is that

1. Okay...the one company that has a homogeneous IT environment, you don't need SOA.
2. Okay...some specific robotic control systems don't need SOA
3. Okay...hmmm...I guess those folks who are still using Multiplan and Wordstar don't need SOA
4. Okay...when you specifically don't want any of your systems to integrate with any other systems then you don't need SOA.

So all six of the people who the above rules apply to - they don't need SOA. 

The rest of us should probably look pretty closely at SOA.