For an excellent and in-depth look at the salary cap and what is and is not included then have a listen to the Eggchasers rugby podcast this week (the domestic mid-week one rather than the 6N one).
Their conclusion is that it is VERY difficult to breach the cap and that there are good reasons why players may be earning less at Saracens than they could elsewhere.
Not saying that's definitely correct but it made me think again about the issue.
I've just listened and they completely missed the point - of course it's easy to police the salary cap if everyone declares all their expenses. However if you don't record the expense then there's plenty which could be excluded.