Try this:
create table tbl1(id int primary key, dt datetime default current_timestamp);
Background:
The DEFAULT constraint specifies a
default value to use when doing an
INSERT. The value may be NULL, a
string constant, a number, or a
constant expression enclosed in
parentheses. The default value may
also be one of the special
case-independant keywords
CURRENT_TIME, CURRENT_DATE or
CURRENT_TIMESTAMP. If the value is
NULL, a string constant or number, it
is inserted into the column whenever
an INSERT statement that does not
specify a value for the column is
executed. If the value is
CURRENT_TIME, CURRENT_DATE or
CURRENT_TIMESTAMP, then the current
UTC date and/or time is inserted into
the columns. For CURRENT_TIME, the
format is HH:MM:SS. For CURRENT_DATE,
YYYY-MM-DD. The format for
CURRENT_TIMESTAMP is “YYYY-MM-DD
HH:MM:SS”.