"Real" DVD authoring programs allow you to create a menu with more than 2 cells, which facilitates this type of thing and helps make the transitions more seamless. Ok, DVDA4 isn't too bad, it's just missing a few helpful items to do this.

As DVDA only allows menus with 2 cells, you can create an intro and a loop, but no exit for a transition and so you have to do some playing.

Basically, rather than connecting your menu button directly to the video that would normally be displayed, you connect it to a transition movie, which in turn connects to the video to be played via the end action of the transition clip. Since nothing is seamless, such as transitioning to a different cell in the same menu or to another video clip, I like to start my transitions with something abrupt, such as a screen of white or black, unless I'm dealing with a fairly static background on my menu in which case my transition clip will start with a snapshot of that background. Same deal with audio - I usually try to contrive something to cover, such as the transition starts with a wave crashing on the beach, a strike of lightening, the gong of a bell, whatever.

So, create your transition clip, drag it onto the project view rather than onto a menu and change end actions or linkages on buttons as appropriate.

--Scott