I can't find many, but of course there are more than a few rowers who have spent time on a bike training, and I suppose it would be a stretch to mention all those hours coaching on a bike!
They are admittedly weak, but here are a few cycling/rowing connections. Can you help with more?
People Rowing and Cycling
British Olympic Silver Medallist Rebecca Romero competed in Athens in the Quad and has switched to cycling and has been offered a spot by the British Cycling Federation fast-tracking athletes for the 2008 games.
Jake Wetzel (Athens Silver in the Four for Canada) was a member of the Canadian Junior Cycling Team.
Marty Aitken was a professional cyclist before going into rowing and then coaching the GB Eight and lightweights.
Amy Gillette, former Australian Olympic Rower, was tragically killed last year while training in Germany as an international cyclist.
Miscellaneous Connections between Rowing and Cycling
Oxford University does a Tour de France training camp, exchanging boats for bikes and riding some of the toughest parts of famous Tour stages.
Tour de France diary (2004) by Lance Armstrong's coach Chris Carmichael discusses cooling vests for "pre-cooling" before hot stages. Mentions the Australian Rowing Team having experimented with them.
Scientific Studies on Rowing and Cycling
"Results indicated that energy costs for rowing ergometry was significantly higher than cycle ergometry at all comparative power outputs including maximum levels."