Mac OS X 10.4 added the capability to fill out PDF forms in Preview. However, if you hit Return while entering information in a form field, Preview de-selects that field, so that you can't enter any more data. If you want to enter more than one line in the field, there isn't a documented way to do so. Contrast this with Adobe's Reader, where hitting Return just gives you a carriage return and allows you to continue typing on a new line in that field.
I discovered that you can, in fact, enter multiple lines in a field in Preview. You simply need to hit Option-Return instead of just Return. This does not seem to be documented in the help file.
[robg adds: I think Option-Return may be a standard Cocoa feature, as this same trick works in iChat to send a multiple-line message.]

