It will depend on the support authentication type by the web server. If the web server only supports BASIC authentication, the BES will be more than happy to send along Base64 encoded version of the users supplied username and password. This is no different than
any desktop web browser. It is important to note that this is considered secure
only in event that the session is secured by encryption (such as SSL).
The MDS Connection Service also supports other, secure means of authentication without the use of SSL (again, just like a typical desktop web browser), including Kerberos and NTLM (which you would be using most likely for Integrated Windows Authentication with Internet Explorer and Sharepoint).
NTLM - Wikipedia, the free encyclopedia
Kerberos (protocol) - Wikipedia, the free encyclopedia