Congrats. Although the letter you will get will make it sound as if you've only "progressed to the next phase of the process" or something similar, you are now what they call a "pre-hire."
This means you will be hired at some point, dependent almost totally upon the economy and aircraft deliveries (and perhaps retirements, but only when the retirement numbers start jumping up in a few years). There are only a few ways you would NOT actually become an employee:
1) Fail to get your type rating. Never heard of anyone that this happened to.
2) Fail to complete company training course (sim check). This would require an extremely poor attitude, or multiple failures of the checkride (a single failure or requiring extra sim training is no big deal). If you study what they tell you to, they will get you through the course.
3) Fail to complete Initial Operating Experience. This occasionally happens, I guess, but they will give you two shots at it, and possibly additional training in between, and they're pretty patient, (i.e. they expect a certain level of buffoonery on your first couple of trips) so again, if you do a reasonable amount of study and preparation, you'll complete it successfully.
4) Fail the drug test or background check. If this happens, you probably already knew it was going to, and you know who to blame
5) Economy goes in the tank so long that they lose confidence in the currency of their info on the poolies, and have to start reaccomplishing stuff. My gut feeling is that we'll see "some" additional hiring this year yet, but I'll bet your "pre-hire" status is good for at least a year or two.
In other words, whenever you get correspondence from the company, remember that it was written by lawyers, who don't want to give anyone grounds to bring suit when things don't go the way someone was hoping they would. You've already completed the most difficult part of the process.
