This is a perennial classic and definitely very popular with cyclists from around the world. The main draw is the stunning landscape, combined with some unpredictable weather and bad roads makes this a challenging but yet a very rewarding journey.
The route is around 600km long and cuts across some of the greatest mountain ranges in the world. At Rothang La (3950m) you'll be crossing the Pir Panjal range, at Baralacha La (~5000m) you'll be crossing the Great Himalaya, and you'll have to go through Nakeela (~5000m), Lachulung La (~5150m) and Taglang La (~5300m) inorder to finally cross the Zanskar range. Barring Rothang La the roads leading to the other passes are all in good shape (for cyclists, as of Oct 2010) and you should not encounter any issues.
For those of you who cycle light, you need not worry about food and accomodation on this route. There are plenty of makeshift restaurants and camps (called Parachute Tent Dhabas) available on this route. Basically you'll find one such camp just before and just after crossing each of the high altitude passes.
List of food and accommodation stops on the Manali - Leh highway.
For those with some extra time, i suggest you take the diversion at the More Plains and go towards the Tso Kar lake. The lake is only 8-10km away, you can easily spend a day there and go back to join the main route towards Leh. But you can also keep going in the other direction to reach the Indus river and cycle along it. This diversion adds about 120km to the overall trip but IMO its well worth it. Especially if you visit the Tsomoriri Lake. The alternate route is marked in the above map (the dark brown one).
The best time to do this trip would be during the 2'nd week of September. The rains would've stopped by then and you'll have clear blue skies and dry roads to traverse. During the rainy season there would be a lot of water on the road and you'll have to traverse a lot of little river crossings. Sudden flash floods are a common occurence during the rains. So do play safe.
Finally a word about doing this route in reverse. i,e ride from Leh to Manali. Some people seem to think that this will be a easier route since you'll be going from a higher altitude to a lower altitude. But based on what i've seen on the route this need not be the case. While going from Manali to Leh, the ascent to each of the passes is long and gradual while the descent is usually sudden and steep (atleast it felt that was for me). So going in the backward direction, you'll have to expect some pretty tough climbs.