The question the senate wants to put to the country is;
'Do you agree with the continuation of the process of change led by President Evo Morales Ayma and Vice President Alvaro Garca Linera?'
If 51% of Bolivians answer "no", then a general presidential election will have to be called.
A really nasty, sly piece of back door politics by the rich white racists that populate the senate, and it will be interesting to see how the president responds to this. He does not have to ratify the vote; if Morales prefers, he is totally within his rights to ignore the call for a national referendum. However, he may decide to go along with the idea and face his nation on what is in essence a confidence vote. If he won it would shore up his mandate in a somewhat definitive manner. With a simple majority in his favour, his position would be considerably strengthened in the debate over the separatist movement in Santa Cruz and other states.
On the other hand, he may decide not to ratify the senate vote and ignore the calls for a referendum, calling it the piece of sneaky underhand politicking that it is. Whatever happens, the ball is now in Evo's court.
UPDATE 10pm: Evo is going for it! Morales has accepted the challenge and his presidency will go to a national referendum. In his words, "If the politicians cannot agree, it's better that it goes to the people." Can't fault the guy for guts, and if his current 56% approval rating holds good, he'll have played a mighty strong card for the future.