It seems there was sufficient response to the shutdown announcement on the Xmarks site that thye have opened up a pledge database to see what the numbers are like. If they get 100,000 pledges of willingness to pay between $10 and $20 per YEAR, they will consider not shutting down the servers.
Go to: http://www.xmarks.com/about/company
And make a pledge if you too would like them to continue to offer their service...