datetime format changed between mysql 5.0 and 5.1?

Mysql 5.1 manipulates microseconds as the same way mysql 5.0 do, however microseconds cannot be stored into a column of any temporal data type. Any microseconds part is discarded as per Mysql Doc: http://dev.mysql.com/doc/refman/5.1/en/datetime.html. However, since you are getting error with this, you can enable allow invalid dates using the sql mode setting in mysql.

sql-mode=allow_invalid_dates

in my.cnf or my.ini (depending on your OS).

http://dev.mysql.com/doc/refman/5.1/en/server-sql-mode.html#sqlmode_allow_invalid_dates

PS: It is surprising that the date & time you provided in your question is actually a non-existent time in DST (14th March 2010):)