Like the others said - don't get too excited just yet. You will still need to keep a close watch on your CR and your mailbox.
On the plus side, they may have deleted the entry in the credit report because they couldn't find your past due account in their system. Sometimes this can happen if a company converts their billing system and accounts fall through the cracks. If they can't find the original documentation that you owe this, then they sent a request to the CRA to delete the entry because they can no longer verify it.
On the minus side, they may have sold your account to a CA and you'll probably start hearing from them instead.
In all honesty, if they deleted the entry instead of sending you a letter agreeing to your terms, I would bet that they lost your account and they can no longer verify that you still owe the debt. (If it was a PFD, they wouldn't delete the entry until they receive your money)
Even so, I would still watch my CR and mailbox just in case.....