From the manual for TikZ and pgf, version 2.10:
In Section 16.4.3 Text Parameters: Alignment and Width for Multi-Line Text
\\ inside your node to mark the end of lines and then request TikZ to arrange these lines in some manner. This will only be done, however, if the align option has been given.
You can request that TikZ does an automatic line-breaking for you inside the node by specifying a fixed text width for the node. In this case, you can still use
\\ to enforce a line-break. Note that when you specify a text width, the node will have this width, independently of whether the text actually “reaches the end” of the node.