write.table writes unwanted leading empty column to header when has rownames

Citing ?write.table, section CSV files:

By default there is no column name for
a column of row names. If col.names =
NA and row.names = TRUE a blank
column name is added, which is the
convention used for CSV files to be
read by spreadsheets.

So you must do

write.table(a, 'a.txt', col.names=NA)

and you get

"" "A" "B" "C"
"A" 1 4 7
"B" 2 5 8
"C" 3 6 9