The problem is that Ubuntu 11.10 supports multiarch (32 bit packages on a 64 bit architecture) but aptitude does not.
As a temporary solution you can comment "foreign-architecture i386" in "/etc/dpkg/dpkg.cfg.d/multiarch" to be able to use aptitude (and uncomment if you need to install 32 bits packages)
The problem is that Ubuntu 11.10 supports multiarch (32 bit packages on a 64 bit architecture) but aptitude does not.
As a temporary solution you can comment "foreign- architecture i386" in "/etc/dpkg/ dpkg.cfg. d/multiarch" to be able to use aptitude (and uncomment if you need to install 32 bits packages)