Pictures appear to go on the SD card by default. At least mine appear to and I didn't change anything. As for music, I'm not sure how you're getting the music on the card, but I have some on mine and the media player appears to pick and play it just fine.
As for the apps, I think this might not be possible just yet? Anyway sorry I just got my phone 2 days ago and I'm a babe in the woods with this thing .
I too am a new user but I did much hacking as of yet. First is I did a manual install of 1.5 cupcake. I don't know if they started sending ota in the US yet and if it makes a difference for saving stuff to the sd card, but one way you can organize and add to your sd card is plug it into your computer and mount it. Plus I just downloaded a great little app that is called ASTRO, this gives you the ability to search and manage folders on the phone and sd card. I am still trying to figure out how to have it set up to automatically save and install the apps on the sd card instead of the phone. What I have found out is that some of the apps saved data is on the SD but the main app is on the phone.
I am not going to be much help other than the pictures i take go straight to my card. The music went to the card after i usb connected my phone to my pc and downloaded the music. As far as the apps go i would love to be able to place them on the card so my phone resources stay powerful. I have had the Eris for two weeks and it's the best phone i've ever used to date.