In Qt v4.5 and earlier: No, the order is undefined as can be seen in the documentation here:
If several slots are connected to one signal, the slots will be executed one after the other, in an arbitrary order, when the signal is emitted.
Edit: From version 4.6 onwards this is no longer true. Now the slots will run in the order they are connected. The relevant paragraph of the current documentation:
If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted