I believe RAID10 support (through the fancy all-in-one RAID10 driver rather than doing a RAID0 of RAID1s) is not present in the installer for Etch or Lenny, but it has been added for the current development version so will be in the next release (or available now if you don't mind using the testing distribution, but this is not recommended in a production environment).
For now you should be able to construct RAID10 arrays in the installer the traditional way, i.e. a nested RAID arrangement, by creating a pair of RAID1 arrays and then adding them into a RAID0 array - though you obviously won't get the all-in-one RAID10 driver's extras (3 device array support, arrangements that can improve read speed for some I/O patterns, and so on) this way.
IIRC this is the same for Ubuntu's alternate installer (the standard installer doesn't offer RAID options at all). I only remember seeing 0, 1, and 5 as options when I installed 9.10 into RAID arrays on my netbook.
In any case you need to make sure that your
/boot filesystem is not on RAID10 (or anything other then RAID1 or a plain volume for that matter) as Grub can not boot off RAID other then RAID1. You should be OK to have your root filesystem on RAID10 though.