[Update: Though the precise nature of the cardiac event remains unclear, Rybak has now taken to Twitter from the hospital to assure friends and supporters that he is okay.]
Other local news outlets have reports up but with very limited information. The only common thread is that Rybak was apparently cross-country skiing with his wife when the event occurred. Here's kare11's report on the web.
Late Update: Tribune Reporter Eric Roper reports on Twitter the City Council President Barb Johnson said Rybak was "talking with his family following the incident" which would be a positive sign. But every report at this point should be taken as subject to revision. Other reports speak only of "chest pains" but the Kare11 tweet-report above seems most specific, detailed.
Late Update #2: The Strib has released their first published report. Gist is that he's hospitalized after chest pains, reportedly talking to family members, according to city official. No more details on severity of the event.