Impact of loss of unique SERIAL?

I am new to these lists and this message may be off-topic for this list. If
so, please advise and I will repost to one of the others. Here is the
question:

We have been running Postgres 7.2.x for our website, and are considering the
upgrade to 7.3.x. However, in reading through the changelogs I notice that
SERIAL values are no longer unique. Now, I'm no SQL guru, but I'm wondering
what happens if two INSERTs are done at the "same" time (we have multiple
web hosts that access one large database server). Would they result in two
identical values on the SERIAL? We haven't gotten a stupendous amount of
traffic, so I don't know what the odds of this are, but I'm wondering why it
was removed.

I really wanted the "DROP COLUMN" command which is available in 7.3.x, but
it seems I'm going to have to remember some really arcane code to add the
UNIQUE to the sequences. I'm wondering how much reworking of my code I'm
going to have to do to upgrade, and whether I should postpone this.
Evaluating the impact of the SERIAL change would be helpful to me, if anyone
could post a response.

Thanks,
Justin Long
www.strategicnetwork.org