I was trying to compile xen-utils-4.1 on Squeeze:
$ apt-get source -t testing xen-utils-4.1
$ pbuilder --build xen_4.1.2-2.dsc
$ dpkg -I /var/cache/pbuilder/result/xen-utils-4.1_4.1.2-2_amd64.deb
<snip>
Depends: e2fslibs (>= 1.41.0), libc6 (>= 2.7), libgnutls26 (>=
2.7.14-0), libncurses5 (>= 5.7+20100313), libpci3 (>= 1:3.1.7),
libuuid1 (>= 2.16), libxen-4.1 (>= 4.1.2), libxenstore3.0 (>=
4.0.1~rc4), zlib1g (>= 1:1.1.4), python (>= 2.6.6-3+squeeze3~), python
(<< 2.6), xen-utils-common (>= 3.4.2-4)
</snip>
As we can see, there is a conflicting dependency:
python (>= 2.6.6-3+squeeze3~), python (<< 2.6)