Sorry, but currently there isn't an "open" protocol that we could hook a synchronisation of Freebusy information between Scalix and Exchange into.
Longer term the iCal standard "may" provide a method for Freebusy exchange between hetro-geneous systems, see Section 4.6.4 in:
http://www.ietf.org/rfc/rfc2445.txt
Personally I've not looked in detail at this area and on a quick glance it isn't clear to me how such a request/reply exchange would occur, between either users or servers. In either case the client and/or server would have to support this mechanism. Scalix does not at this point in time.
The Outlook Client does provide the ability to publish Freebusy information to a .vfb file. I've never tried this and you have to have some way to tie user information to the URL/web/file server where the .vfb file is published to enable user lookup.
This might provide you with something to fill the gap, but this isn't something we have either specifically tested at Scalix or something that we'd generally recommend.
Cheers,
JG