Ditto what michigan said, except the part about impressing girls - I prefer to intimidate the boys! :p
I lift 2 - 3 days a week depending upon my schedule. Cardio work on the other days.
I work on building strength over the winter and try to just maintain it when we can ride more. I'm using a superset routine now and it's quick and I feel like it's working 3 sets of 8-12 reps for 1 - 2 excersizes of opposing muscles back to back. Upper body one day, legs one day, combo one day if I can and cardio on the other days (or snowmobiling).
I have lifted weights for sports since I started in sports a long long time ago and feel that it helps with riding, picking the bike (or sled) up, protects against injury, helps in recovery, speeds up the metabolism, increases self-confidence and self-reliance, I could go on and on and on.....
I just set down rules for my 9 year old son to start doing a workout at Grandma's when he gets home from school - he needs to catch up to his little sister before she really kicks his butt. No weights but he has to bicycle or walk or run for at least 20 minutes and do 15 situps and 15 pushups.