Hopefully this will get you started with movies on your Treo.

Step 1. Get the VOB file from the DVD
Use DVDDecrypter v3.5.4.0 from http://www.mrbass.org/dvdrip/
read more on http://en.wikipedia.org/wiki/DVD_Decrypter
Follow instructions on www.doom9.org 
From the menu on the left select Guides, ripping, DVD Decrypter guide
About half way down the page follow instructions for --> IFO mode
a. start DVDDecrypter, then put the DVD into your pc
b. click on file folder if you want to change location of output
c. input tab is usally fine
d. click on Stream Processing. uncheck everything but first 2 checks (Video and audio)
e. click on the DVD icon
Takes about 20 -40 minutes depending on speed of pc
(I have dual Core, 2 GHZ, 2 GB memory - 20 minutes)

Step 2. From the VOB create the file for the Treo
http://www.pocketdivxencoder.net/
closest size is PDA - 320 x 240
1. select the file (click on the pc icon)
2. select output file location
3. increase brightness and sound to 10%
4. RESIZE (optional)
most movies are now widescreen which is not good for the square Treo
so you end up with 320 x 180.  I usually go to RESIZE and expand the movie, 
losing the edge of the movie.  I usually go to 320 X 192.
If you do get a movie that's not widescreen then you will actually end up 
with movie that fills the Treo screen.
5. click Encode Now.
Takes about 20 - 40 minutes.

optional
www.divx.com - plays the movies you made on your pc (though they are same size as on the phone )