While I see your point Guido Preite , I disagree :)
First, Microsoft owns all of its infrastructure so this can easily be done technically.
Second, all customers of Microsoft using these domains (dynamics, onmicrosoft etc) are businesses so they have a need to protect their brand, company name and trademarks and have the means to do so.
Third: it is common to do exactly this in other areas. Google AdWords for example does not let you use registered brandname/trademarks unless approved by the hold of such trademarks.
And last but not least: it makes a lot of sense from security perspectives too.
I won't try it but if you would register pepsi-sso.crm4.dynamics.com, I am quite sure things will turn ugly quickly.
Pete
