Sequences

Hi All,
I am using a sequence to automatically generate a transaction number in
my database. I've set up the transaction number to have a default value
from the sequence, so that I don't have to provide a number while
entering. However, I want to know the number of the transaction I've
just entered. I can do that by calling curval(sequence), but in a
dynamic situation with lot of transactions being recorded by multiple
users, how can I be sure I am getting the value which I had entered.
It sounds silly, but I am sure there is a simple solution for it. Do I
have to lock the sequence while I am using so that no one else can use
it, or do I fire both the queries in the same TRANSACTION block?
Thanks for any help,
Anand.