I am taking a bike tour from Vienna to Prague in July '10. I'm flying round trip to/from Vienna. What is the best way to get back to Vienna? Do you need to reserve a spot on the train from Prague? How often do they run?

There are five direct fast trains a day from Prague to Vienna. It will take 4 hours 44 minutes. Prices are very reasonable. Some of the trains have places for bikes but you should reserve this in advance. I'm not sure whether the other trains have space but you don't have to book, or if there is no room. Anyone else know? The timetable info is available at
http://jizdnirady.idnes.cz/vlaky/spojeni/
There's a union jack in the bottom right hand corner if you need the information in English.
Good luck with the ride!
http://www.jeremytaylor.eu/prague_photos.html

If you have the time available, I recommend a night in Cesky Krumlov on your way to back to Vienna.

It's a beautiful UNESCO World Heritage site! And it's castle has one of the two remaining baroque theaters left in the world... very interesting to visit (as is the regular castle tour)!

Cesky Krumlov is a delightful place though will take you a little longer to get to Vienna. Train from Prague to Cesky Budejovicka - you could then cycle the 28km to Cesky Krumlov. Back to Cesky Budejovicka and then a train to Linz and from there to Vienna. If you have the time, go for it.
http://www.jeremytaylor.eu/cesky_krumlov_photos.htm

Thanks for the train schedule! Cesky Krumlov is already on our itinerary with a kayaking option.