Remember, you have to set the snap tolerance of each layer individually for each layer you want to edit. To do this in 0.11.0 Metis, select Settings -> Project Properties and click “Snapping options…” under the “General” tab. Make sure the checkbox next to the layer you want to edit is selected. I have not needed to change the tolerance from “0” yet. This is set in map units (see Project Properties, General tab).
Really, this is an annoying step, and something that probably frustrates noobs like me (who jump in without reading the whole guide) a whole lot.
Source: QGIS User Guide for 0.9.1 Ganymede, Chapter 4.4 Editing (about page 47 of the PDF). This PDF should have come with your install.