Getting sequence value after inserting many rows at a time

Hi,

I have a PHP script that do something like this:

INSERT INTO table_with_sequence (field1, field2...., fieldn)
SELECT field1, field2, ..., fieldn FROM table,table
WHERE condition, condition

My doubt:
Will the rows just inserted in "table_with_sequence" always be sequencial?
(won't it happen to have X rows from this INSERT, 1 rows from another INSERT
in the middle and the rest rows of the first INSERT?)

How can I safely get the first value the INSERT "generated" for
the sequence?

[]'s Mauricio