I have the same problem (Ubuntu Lucid Beta 1, alternate installer).
I created two RAID1 partitions on two disks, one for boot and one for LVM. Then I formatted the boot RAID parition with ext3. After setting up LVM was done and I completed the parition creation, the installer popped up the following error:
"The attempt to mount a file system with type ext3 in RAID1 device #0 at /boot failed."
I have the same problem (Ubuntu Lucid Beta 1, alternate installer).
I created two RAID1 partitions on two disks, one for boot and one for LVM. Then I formatted the boot RAID parition with ext3. After setting up LVM was done and I completed the parition creation, the installer popped up the following error:
"The attempt to mount a file system with type ext3 in RAID1 device #0 at /boot failed."