I think you need also need to boot the updated zImage kernel image.
The kernel image contains some extra symbols for dvb-support.
Try this on your N900:
1. enable extras, extras-testing, extras-devel
2. apt-get install multiboot multiboot-kernel-maemo kernel-power kernel-power-config kernel-power-modules
3. Create an entry for kernel power: /etc/multiboot.d/00-kernel-power.item:
ITEM_NAME="Kernel Power DVB"
ITEM_KERNEL=2.6.28.10power46
ITEM_MODULES=ext3
4. Now copy your modules: dvb-core.ko, dvb-usb.ko and dvb-usb-dtt200u.ko onto your N900 (into the right directories).
5. Issue a: depmod -a
6. Now in your scratchbox enviroment go into your kernel dir and type: make make EXTRAVERSION=-omap1 zImage
There should be a file called "zImage" in arch/armel/boot or something (just look for it:P), copy this file to your N900 in /boot/multiboot and rename it to: "vmlinuz-2.6.28.10power46"
8. Reboot your phone and select: "Kernel Power DVB"

Now some notes.
1. Be sure that your scratchbox enviroment is set to ARM and not X86 (you cant run the graphical emulator in ARM-mode but you dont need it anyways).
2. I think EXTRAVERSION should be set to "power46" instead of "-omap1", so it matches the kernel and modules installed on your N900.