The LWG just covered this scenario see (item 7)
https://docs.google.com/a/osmfoundation.org/document/d/1uemhXWKwbu3RNjAWcG0R-nEFaN1FHjZl1McFwjXkNSc/pub
further reference ODbL definition of "publicly":
*“Publicly” – means to Persons other than You or under Your control by either more than 50% ownership or by the power to direct their activities (such as contracting with an independent consultant).*
General note: this is not a free pass to circumvent the ODbL, it just shifts the responsibilty for access to the derivative database to the customer of the SAAS service. If for example the customer publicly uses the data the normal ODbL terms would apply. Further note Note it further assumes that the SAAS provider is using essentially unmodified OSM data (see trivial transformations), if he is actually providing a derivative database as a base for his service that would naturally be publicly used.