I have been asking the same question. As best as I can tell, the previous versions of MS Word (and open for read only protection) had some flaws that allowed people to easily crack passwords. Those same hacks do not seem to work on 2010. (this is only about the password to open the document – I am not talking about the read-only password, which is kind of an oxymoron). Likewise, a search of the internet seems to show that many of the decryption programs do not work on MS Word 2010.
With that said, brute-force methods will always open any file, given enough time. What once took a supercomputer months or years to do can now be done with a home laptop. So today's password might be vulnerable to tomorrow's brute-force power.
Summary: At the moment MS Word 2010 seems sufficient for most business purposes, where you can easily make the password so complex that the brute-force methods would take too long for any reasonable adversary. (P.S. Always assume that people in government agencies will be able to crack any encryption they want to, because they have access to massive computing power)