most of use fondant the cake with a chilled cake... I have a spray bottle and it's set to mist.... I mist the cake with water then put the fondant....
as far as the air bubble Kita.... did you read leah's thread on "burping" a cake?
her tech. helped me a lot and now I don't have any air pockets.... I used to have to use a pin to deflate them.... but now I don't have to.... it's a good way to make sure this will not happen....
hth
I think so, where you put a weight on the cake? I wasn't sure what I thought about that.... certain fillings are sure to ooze out...???? plus some of my cakes are pretty spongy and would smoosh....
I actually came to this forum today looking for an answer to this particular problem. I had the same thing happen to a cake that I decorated last night. I also cover my cakes with fondant while their still cold. And I was so proud of myself because I'm getting so much better at covering a cake very smoothly all the way down to the bottom. So both tiers look beautiful and I'm decorating away and all the sudden I notice bubbles. Not just one but several. I kept trying to smooth them out and even poke a toothpick in a couple spots that I knew I'd be putting decorations on. Well I finally had a huge blowout in the back of the cake and ended up putting a fondant "patch" over it. And my fondant started sagging really bad and kind of pooling at the bottom of my tiers. I was actually blaming the type of frosting I used under the fondant. It wasn't a crusting buttercream. When it comes to room temp it's actually very runny and "melty". Maybe my sagging came from that? The bubble thing could have been something entirely different. It's just so sad because you're so carefull and patient to make sure the fondant covers nicely and then it just messes up anyway 
...It wasn't a crusting buttercream...
The buttercream I generally use under fondant is not a crusting buttercream. There's no reason for it to be. The only time I use a crusting buttercream is for a non-fondant covered cake. Or, if I want buttercream flowers and borders to have a hardened finish that is less likely to be smooshed or stick when touched.
I never have sag or bubbles.
What recipe is your frosting? It being runny at room temp could be one of your problems.
I do exactly the same as artscallion: I crumbcoat the cake, put it in the fridge and let it stand there for at least an hour. I then take it out and cover it immediately with fondant. I work from top to bodem, smoothing out the fondant with the palm of my hand and with smoothers.
I ALWAYS let it stand over night at room temperature. In the first few hours, I take a peek once and a while. If air has been trapped, it are usually small pockets. I stick a tiny needle in the pocket and carefully push out the air with the smoothers. After a few hours you can tell if there are airpockets left or not. Next morning, my cakes are always clean and neat, no bubbles at all. Just the way I want them.
Poking a hole in a cake with a straw would be an idea, still think it is a little scary! Having a big hole in your fondant....
I ALWAYS let it stand over night at room temperature. In the first few hours, I take a peek once and a while. If air has been trapped, it are usually small pockets. I stick a tiny needle in the pocket and carefully push out the air with the smoothers. After a few hours you can tell if there are airpockets left or not. Next morning, my cakes are always clean and neat, no bubbles at all. Just the way I want them.
....
I do the same thing, because if you let it stand for a while you'll be able to tell if a bubble is forming.
Air coming from inside is what causes the bubbles, so it could be caused these ways:
1. Warm cake cooling and letting gas off
2. Small pockets of space in the tier between layers, then the weight of the fondant and the smoothers force the air out while the cake is being covered.
3. Fondant that's too long on the sides, then when the smoothers are used it forces the side fondant down and it has nowhere to go when it hits the borad, so it will stretch out not down.
4. Buttercream underneath melting and creating a little "give" and sagging.
Basically everything that's been suggested already 
You just have to do what mireillea does and keep an eye on it while it's setting up.
Incidentally, you can have blowouts on icing, too. The only time it's ever happened to me after the cake was delivered was when I was delivering in a snowstorm and had to go over some train tracks fast or get stuck, so I hit them pretty hard, and it created enough stress on the cake that it developed a tumor later. I only found out because the photographer sent me a picture of the cake, and I noticed it. She said that she hadn't noticed it, but I did!
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