Yep it's doable. See http://www.linuxdoc.org/HOWTO/Boot+Root+Raid+LILO.html
It depends on the RAID controller though. Some want you to everything at the BIOS level. For some hardware that plays nice with Linux check out www.3ware.com.
One of the bodies on this board has been using them with very good results.
A po' boy like me hasn't had the opportunity to play with one of 'em yet, but I can do it with software RAID no prob. The procedure goes something like this. (PLEASE READ THE HOWTO for a detailed setup!!!)[list=1][*]Confirm you have the RAID as part or your kernel (or module if you want to make things more exciting) and that you have the most recent version of 'raidtools' installed and conifgured.[*]Shut down the server
[*]Install new hard drive.[*]Power up and partition.[*]Build your /etc/raidtab file - mark the ORIGINAL disk partitions as 'failed-disk'[*]Build the RAID array - 'mkraid /dev/mdX' since the ORIGINAL disk is marked as a 'faild-disk' it won't get striped (yet).[*]Format the array 'mke2fs' or 'mkreiserfs' ( I like reiserfs myself...)[*]Mount the array[*]Copy your current root fs to the array (I use this from the / fs 'find . -xdev | cpio -pm /where_you_mounted_your_array[*]Build a boot floppy that uses root=/dev/mdX to see if you can mount the new degraded array.[*]Once you have confirmed that things are working properly you can get LILO to boot from the new array. Test this.[*]Now you can use 'raidhotadd' to include your original disk in the array.