There is a limit on the size of the root partition. Now,
this makes sense in one way because it avoids the issue
where Lilo can't read the kernel because it is beyond the
area of the disk which is accessible by the BIOS.
However, it is possible to create a big root partition but
have a small partition mounted on /boot. The new
installation system stops you doing this.
I am aware that this is a somewhat debatable point, so I
will raise it on the list as well. I for one, though, will
find it very annoying if it gets into the final product.
(If you do decide to put it in, it would be nice to have a
way of overriding it, for example by stopping it applying to
expert mode installations.)
Disk Druid merely throws out a warning. It allows you to do this, but
you must later put in a smaller /boot partition. The documentation
(and possibly the disk druid warning) could be a little better here