I don't think you can qualify that as a threat. certainly the landscape is changing, and for the better. i work for a large company that runs it's application from the cloud and for the most part it brings us nothing but joy.

once you start actually using something like amazon DDB(dynamo db-noSQL) or SDB(simpledb) you'll find that it's great for simple things such as scoreboards and lookups, but if you want to perform aggregation, joins and complex queries you have to start using EMR (elastic map reduce) which basically has to perform table scans. DDB only allows you to index the data in 1 way and it's performance is poooooooor whenn you try anything complicated.

RDBMS are still required for complex data crunching and other such functions.

as for NHibernate - well that is just an entity mapping system - it still needs a DB in the background. and if you are going to let your Development team trample all over you and not use stored procs because NHibernate can "write it's own db code" then you need to think long and hard about the implications - if there is some form of database issue at 2am why should you get called out? - put that to your dev team and ask them to be on call - see how quickly they revert to using stored procs.

the RDBMS is not under threat, but we do need to improve our own skills and embrace the advantages of new technologies rather than be staunchly defensive of our 3T systems (Tried, Tested and usually very Tired)