Yes, however the way to achieve it "depends" on how you're deploying
the content. In many cases you'll need to transcode the content if
it's pre-existing in order to apply the watermark.

Windows media encoder (free) can do that using a commercial plugin
(google : instream inscriber) and the Expression media encoder comes
with the ability to apply watermarks built in, though it's not free.

If embedding in a web page, you can literally use CSS positioning to
place an image over the embedded media player - in that case it's
important to add this to your player code, so it overlays correctly by
rendering video directly in the client area of the browser

<param name="windowlessvideo" value="true" />

You can use PNG or CSS transparency (alpha blending) on the image if
required, though IE6 is a bit iffy using transparent PNG images.

HTH
Cheers - Neil
------------------------------------------------
Digital Media MVP : 2004-2007
http://mvp.support.microsoft.com/mvpfaqs

take a look here Bernard http://www.plentipeppa.com  i use inscriber stream
and very happy with it for now and i have not fully explored it.