You'll get your base assigned during class, roughly about the time you start in the sims. Actually, you get 3 bases: your IOE base (or bases), your "temporary domicile" (where you finish the remainder of the month that you complete IOE), and your permanent domicile. For instance, you could do your IOE out of BWI, finish IOE on the 10th of November, fly out of MCO for the rest of November (with a hard schedule given to you by Training Scheduling), and then go to MDW in December as your permanent domicile. Your first month in your PERMANENT domicile will be a hard line, so January would be your first month sitting reserve in this example. Since the hard line in your temporary domicile is a freebie, it's possible to get almost 2 solid months of flying before you start sitting reserve if you finish IOE early in the month.
You put in bids/preferences for IOE base & temporary domicile, but you're really only bidding among your own class. Depending on Check Airman availability (since they fly as your "instructor" during IOE) at each base, who knows what you'll get. Ditto temp domicile; several guys got Dallas as temporary domicile late last year, which obviously they could never hold as a permanent domicile. I think Training Scheduling looks at things like where you live when they assign the temporary domicile. When you get your permanent domicile, you are in the same Vacancy Bid as every other FO in the company, i.e. you get the very last available slots, and it's all by seniority.
In the December vacancy bid, there were about 50 new guys bidding (the second September class and the first October class). The results were as follows: lots of slots to MDW, MCO, and OAK, with OAK going last. The guys in the September class pretty much had their choice of those 3; the bottom half (or more) of the October class all got OAK. Nobody got BWI that month, but at least some guys from those classes were able to get it in January. Also, 3 slots to PHX -- went senior in the September class. None to HOU (still a few months before anybody from those classes can hold HOU) or DAL (junior guy there has been with the company over a year).
Two disclaimers here. First, this is only a snapshot in time; the base that is available in a vacancy bid can change depending on the latest expansion news... if BWI suddenly gets a bunch of slots added to it, your whole class may spend a month or two there (though this would be a bit extreme). Lately, OAK has been the "junior guy hell" base (probably because it's the hardest commute for those living in most of the country), followed by MDW (which isn't so bad, because Chicago is easier to get to & the crashpad situation is great, even though it's mostly a commuter base). MCO seems generally available, BWI and PHX easily available within a month or two at the worst, and HOU and DAL not for a while. But all this can change.
Second disclaimer: the -200 disposition may change some of this. The classes in 2001 were all trained on the -200; the classes in 2002 won't be. That may possibly remove DAL and HOU as IOE bases & temporary domiciles, since they will be the -200 bases & guys starting on the line won't be trained in the -200 (DAL and HOU will still fly -3/5/700, but they'll be the only ones who are kept current on the -200, so any DAL/HOU crew must be able to fly a -200). Or, they may just give anybody who gets DAL/HOU for IOE/Temp Domicile the -200 class right after your checkride & LOFT (the last 2 sims of the class). I don't know how this will play out, but it may have some effect. Short answer, though, it won't matter much for the vacancy bid, since these two bases are distinctly senior anyway.
Hope this helps a bit... if you have questions about all this, feel free to post here or send me a private message.
I received a message from Jennifer this morning saying there will be two classes per month until May. I don't know if this is old news to everyone. I think the classes grew to 24-26 new hires. At our class at HPA in early December, we were briefed seven classes with 20 new hires each from January to April. Looks like things are continually picking up.
To Snoopy58: thanks for the great info. Looks like a commute from OKC to MDW won't be so bad. Any word of advice to prepare for training? We'd all appreciate it.