New, cheap mountain bike: What's most likely to break? I have a very cheap (less than USD $100), new mountain bike I'm about to break in on a 2-4 hour ride on a mountain trail. I'd like to be prepared for equipment failures or problems - can anyone suggest what's most likely to break (or have problems) first?

I already have a spare inner tube (and pump!) to handle a punctures, and expect to have a sore rear end, anything else?

(thanks for the articles warning about cheap bikes - I should probably add that I will use this bike only once a year at most, and can't afford a better one anyway - if not for cheap bikes I'd just be walking. For anyone coming across this question considering buying a very cheap bike, see the link posted in the comments and make sure you know what you're getting into!)

Update:
Thanks for the answers so far. Thought I'd do a quick post-mortem for anyone interested. I went on the ride (Oaks fire trail, very scenic, good fun - Blue Mountains, NSW, Australia). 

I managed to escape without notable failures on the bike, though I think the cheap suspension fork had already seized up and the derailleur only changed gears when in the right mood. 

In our group of 15, we had 3 flat tyres and a broken chain (all on $1000+ bikes, oddly enough - maybe they were just riding them harder). 

The most unexpected problem was with replacing punctured tubes on the expensive bikes. The tube valve hole in the fancy rims wouldn't fit the spare tube's  larger (standard car size) valve, and many pumps couldn't fit the fancy tubes (to re-inflate them once patched).
Information about the specific components would help. I am not sure what will break first, but the derailleurs will probably be the first component to need servicing - especially if you purchased this bike from a sporting goods retailer instead of a bike shop. Brakes will probably be needing some love in the near future. If you ride on trails at all the fork will probably end up needing some maintenance quickly. The wheels will also probably go out of true pretty quickly as well.  

One thing you can do to help keep the bike in good running order is to frequently put chain lube on your chain to keep it, your derailleurs, and cassette well lubricated.