It is not a full-blown DLNA solution, like Samsung's AllShare app for Android, for example, since you can't receive media to your phone, but for quickly watching an HD movie on your TV without all the cable hassles it does a fine job. Interestingly enough, the interface of the app is full-blown Symbian Anna, rounded icons and all, so the official OS update for Symbian^3 devices might be just around the corner.
As with all Wi-Fi streaming apps, the phone and DLNA gear, like TV or audio system, have to be connected to one and the same Wi-Fi network, preferably of the fastest variety possible. Otherwise you'll be waiting for a while for large high quality videos to start streaming, and look good in the process, for which Nokia Beta Labs gives a bit of warning, and also notes that the interface is not finalized. Watch Nokia Play To in action in the promo video below.