Yeah, you don't have ext3 support compiled into the kernel. Having it as a module naturally has to result in a kernel panic: The kernel starts and wants to mount the root partition. That is ext3. ext3 support is in a module that resides on the root partition -> panic!
What you can do to start nonetheless is to change the fs-type entry in /etc/fstab for the root filesystem back to ext2. ext2 and ext3 are compatible so that a cleanly unmounted ext3 filesystem can be handled by the ext2 driver.
Now you either compile your kernel with ext3 support, (apt-get doesn't compile the kernel, it download and installs packages) or you apt-get the proper kernel that has ext3 support compiled into the kernel. Try apt-cache search kernel-image to see what's available. Some package description should mention ext3 support.