I've looked at this issue some time ago -- and I'm sure others may
correct me -- but I am under the impression that the meaning is quite
vague here. Although the normal "rule" is that an aorist participle
means action prior to the main verb, that does not often apply when the
main verb is in the aorist as ELABETE is. So, it may be either.
It may be difficult to avoid theological issues with this verse as it
is a proof text for those (mainly Pentecostals and charismatics in my
perception) who view "receiving the Holy Spirit" to be a distinct (but
not necessarily separate) event from (water) baptism.
This verse also shows up in criticisms of the KJV's translation ("since
ye believed"). Part of the criticism is somewhat misplaced because one
of the meanings of "since" in the 17th century was indeed "when." Also,
"since" has an atemporal meaning, "in view of the fact that," so "since"
is no more precise than the Greek.
-- Stephen C. Carlson, George Mason University School of Law, Patent Track, 4LE email@example.com : Poetry speaks of aspirations, and songs http://osf1.gmu.edu/~scarlso1/ : chant the words. -- Shujing 2.35