I've posted up a schedule of the Rio samba schools technical rehearsals in the sambodromo in 2011.
Carnaval is really late this year, at the start of March, so the rehearsals start late too, in January. They are taking place every saturday and sunday night. Entrance to the sambadromo is free, and the rehearsals are always very popular so get there early to get a good seat. Usually theree is one samba school from Grupo A and two from Grupo Especial per night, followed by a show in front of sector 3. The samba schools parade without their carnival costumes or spectacular floats, but they usually have some treats for the crowd, and its actually easier to see the dancing and choreography without the visual overload of carnival itself.
The list of rehearsals is here: http://www.tdsounds.co.uk/rio-carnival/rio-carnival-2011/rio-samba-school-technical-rehearsals-2011.

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.