Henrique: I've got good news and bad news.
The good news is that if your users are using IE on Windows and Ex2003, they can go to the OWA options page and download + install the S/MIME ActiveX control and they'll be able to sign/verify and encrypt/decrypt S/MIME messages.
The bad news is that in OWA2007 we haven't gotten to adding S/MIME support yet, so you won't see this same capability in OWA2007 for a while.
Christian: I've got all good news for you. The administrator can turn on/off the ability for end users to manage their Exchange ActiveSync mobile devices through OWA using OWA "segmentation". You can do this "per OWA virtual directory" or "per user". To turn off the OWA mobile device configuration for the whole vdir, you can use the Exchange management Console, or execute this Powershell command:
set-OwaVirtualDirectory -identity "owa (default web site)" -MobileSyncIntegrationEnabled $false