I just finished to install Debian sid on a Dell Precision T3400 (I used a lenny netinstall ~ end of 04/2008)
Despite the very good help you can find here Setting up Debian on a Serial ATA RAID device it took me several trials to successfully install lenny (soon to be sid) on dmraid.
Here I'm going to underline my pitfalls and provide a synthetic guide

1. boot with:
   install dmraid=true
2. Create partitions in ATA RAID device.
   Having a /boot partition as the first partition seemed a good idea to make foreseeable problem with grub easier to solve.
3. Write changes in SATA RAID partitioning to disk
4. Finish partitioning and write changes to disk
5. chroot /target; mkdir -p /boot/grub
6. cp /usr/lib/grub/{ARCH}/* /boot/grup
7. grub
   1. device (hd0) /dev/mapper/<isw_XXX_Volume0>
   2. device (hd0,0) /dev/mapper/<isw_XXX_Volume0>
   3. root (hd0,0)
   4. root (hd0,0)
   5. setup (hd0)
8. update-grub
9. vi /boot/grub/menu.lst (the auto generated menu.lst doesn't seem to be OK)
10. go on with installation skipping bootloader phase

First thing I kept on missing was how to activate the Intel BIOS Raid manager.
In the Setup menu (F2) Sata Operation have to be set to Raid On, other "Auto" options won't work. Then you've to press ctrl-i after POST, there you'd be able to create an array.
I went for RAID1.

It could be a good idea to delete all the arrays before recreating one and installing lenny. Especially if you made some previous trial.

Another mistake I did several times was missing the Write changes in SATA RAID partitioning to disk and I kept on selecting Finish partitioning and write changes to disk.

Finally everything seemed to be OK but the first boot failed with grub error 23.

Fortunately it was just a matter to change the root option (press e in grub).
It could be grub automagic menu.lst generation or just the suggested parameters in the wiki were not suited for my setup or whatever (I'm not a grub expert).
Following the wiki grub auto menu.lst generation set root to (isw_XXX_Volume0).
What worked for me was to set it to (hd0,0).
Anyway after a successful boot it was just a matter of changing menu.lst and everything was solved.

I think that if I could just avoid to repeat over and over the above stupid errors, even considering last small glitch, installing lenny on dmraid would be a quite fast and simple process.