Well, a few responses to things on here:
The one piece of advice I know on places to avoid on Derby night would be anything in the downtown area going towards the West End. "Derby Cruising" was a tradition for what seemed forever, in which hundreds of cars would cruise up and down Broadway and making the entire stretch a stand still. It's been banned for a couple of years technically but traffic is still a nightmare. Even ambulances can't get through too easily! Also anything close to the Downs itself will be locked up for hours after it's over. Other than that the ladies and gents can't steer you wrong!
The Great Steamboat Race is on April 29th (thing of note for reference, a few of Derby events are based off the calender for that first Saturday in May, such as the Steamboat Race occuring the Weds of that week, the Parade Thursday). Sadly the Delta Queen does look like she is gone this year though I hope by some miracle that changes in the future, it's really not the Great Steamboat Race without her! This year it will be the Belle of Louisville and the Belle of Cincinnati.
Well events that you would miss Apr 21st-26th would be the Taste of Derby Festival on the 21st (it's at Slugger Field, for $75 spectators can sample wares from dozens of local restaurants and there is a silent auction), some of the Million Dollar Hole in One contest events (if golf is an interest), The Mini Marathon, the Great Balloon Glow and Race, and the Texas Hold'em Tournament.
I'm sure these guys can't steer you wrong for the best spots to grab at the actual Derby but one major thing is that unless you are one of the younger crowd looking for one of the most insane parties to ever be found avoid the Infield at all costs. The ambulance service I work for covers all the first aid for the track and that is consdered the War Zone. Those who volunteer to man that area are very brave souls indeed. Just to give a prime example, yes we had to rush not one, but TWO people who after drinking most of the day thought it would be fun to swan dive off the top of a port-a-potty.
As for when tickets will be available, with the exception of the very high end seats according to the website they will not be available until March 2nd. Here is the official Derby website for ticket purchasing though:
http://www.kentuckyderby.com/2009/tickets/buy Ifyou have not done any track betting before, that is a whole new learning experience. Even how you place the bets at the window has to be done a specific way. For a practice run you may want to attend an earlier race day, live racing comes back to the track on April 25th and there will be racing each day leading up to Derby. The folks at the windows are very nice and certainly helped me through my first fumbles at trying to bet!
The official Kentucky Derby website has information on all the events:
http://www.kdf.org/