In a paper I wrote a couple of years ago, I examined the relevancy of the “fallacies of distributed computing” defined by Peter Deutsch almost 20 years ago. Writing about the “Network is Secure” fallacy, I said that after all these years you would think that the fact that you cannot assume the network is secure would be a no-brainer. Alas it still it happens all the time -- and that's for "regular" distributed systems.
In my opinion, assuming the network is secure for an SOA is not only naïve but negligence, pure and simple. The whole premise of moving an organization to SOA is connectedness and integration. So, unless your SOA will fail it will be connected to other systems. Whether you are building RESTful systems, WS-* SOAs, EDAs, or any combination of these architectural styles, if you won’t treat the services boundary as a border and secure it, you will be sorry.
Security in SOA should be considered at the "grand-scheme" level with issues like authertication and authorization, but also at the single service level, looking at issues like DDOS, SQL injection, elevation of privilige and what not. A trivial thing like exposing a transaction beyond service boundaries can translate to an attacker denying services in your system simply by locking out your database. Again, this is just a simple example.
The other thing about security is that you have to consider it early. patching security "later on" can have devestating effects on a system's capabilites especially in areas related to performance. I have even seen military systems that had to go through serious rework, just because security was added as an afterthought instead of handled early on.