Yes, only on the Roku 2 player models.  If a title is available in 1080p, you will automatically receive a 1080p stream provided that your Internet connection has a fast enough download speed, you have a 1080p-compatible HDTV and you are connected to your HDTV via HDMI. Not all Netflix titles are made available in 1080p.

Note: Previous Roku player models will not support 1080p streaming.