The current state of multipath support in Debian Installer's RC2 for Lenny is documented here. The netinst works fine if you use the patched grub (#442382). Unfortunately the CD images are currently broken due to missing libaio udeb.
The easiest way to get the patched grub into your system is via http://tinyurl.com/mpgrub/inst_i386.sh as described in the README.
The recently updated multipath-tools 0.4.8-14 support disks >2TB and also work with kernels more recent then Debian's 2.6.26.