The big parades of the major samba shools of Rio de Janeiro are all viewable on youtube. These days youtube is also showing the more precarious parades of the lower groups in years gone by, and some of these are amazing, for all the wrong reasons. There have been schools that have had to parade without any drummers. Here's Unidos de Campinho, parading in the sambodromo in 1992.
http://il.youtube.com/watch?v=zkkj1gRkSjY
In fact, the drummers were there, but the lorry carrying the instruments hadn't arrived. The bateria (drummers AND drums) is such an important part of the parade that it has its own separate judges. No drums, no marks. The commentary tells us that the instruments turned up eventually, but right till the end of this clip there are no drums to power the dancers, just a cavaquinho and a singer. A particular shame as the song itself was lovely that year. You can see all of the dancers doing their best to participate with energy, but it must have been hard.
This sort of thing tends to happen to samba schools in severe financial and organisational trouble. The following year Unidos do Campinho failed to parade at all due to financial problems, and soon the samba school was completely disbanded.
The video commentator tells us that it happened once to Mocidade Independente do Padre Miguel, when the bateria was being led by the great Mestre Andre. The truck carrying the drums suffered an accident on the way to the parade ground in the Rio Branco. Alternative transport couldn't be organised in time. But Mocidade Independente have continued on to be one of the great samba schools of the special group in today's Rio carnival.

The first Samba School technical rehearsal in the Sambodromo is on the 6th December. Entrance remains free. These rehearsals are enormously popular - in 2009 there was an average of 40,000 spectators at each one. This year these rehearsals will mostly be on Saturday and Sunday evenings; the Friday night events have been curtailed because the necessary road closures caused enormous traffic problems last year.
Saturday 6th Dec also has 3 local football matches , so the rehearsals may be later than originally advertised, to allow the football crowds to disperse down Avenida Presidente Vargas before it is partially closed for the rehearsal.
To find out more about the Rio Carnival technical rehearsals in the sambodromo, and to see a timetable, click here.
A complicated question, on some levels.

I've heard a sneak preview of the Rio carnival official samba CD 2010 (Grupo Especial) and it rocks! Read more here
If you get the opportunity, here's how to get the very best out of your experience.

You can buy a tourist package for Rio Carnival that includes parading with a Samba School. You can also organise it yourself (advice below). Either way, if you want to know how to get the most out of this once in a lifetime experience, here's how to go about it.

Heres a pic of one of the costumes the samba school Mocidade Independente is producing for carnival 2010.