There are a couple LSA's out there that can be for sure used in the IFR environment. One of the Zodiac's 601's can be equipped for IFR flight, as well as the Tecnam P2002, with the right engine/avionics combo. Nothing explicitly prohibits IFR operations, but many of the manufacture's don't write the operating manuals to allow for IFR flight. However, people could try to convince a manufacture to write the manual to allow for IFR operation, but not sure how likely that is to go through.