CBS News has an interesting article that documents the U.N. Economic Commission for Europe and the International Federation of Robotics:

QUOTE
“…said that 607,000 automated domestic helpers were in use at the end of 2003, two-thirds of them purchased that year.  Most of them — 570,000 — were robot lawnmowers. Sales of vacuum cleaning robots reached 37,000.

By the end of 2007, some 4.1 million domestic robots will likely be in use, the study said. Lawnmowers will still make up the majority, but sales of window-washing and pool-cleaning robots are also set to take off, it predicted.
UNQUOTE