I'd agree with the others (but Joe raises a good question as there are a FEW airlines who, due to how they sell their tickets do allow this-- but it's by far the exception and not the rule)...
I think where many people get confused on this matter-- can I skip X leg of my trip? is that what you've bought is transport from A to C... and due to the airlines schedules or the like, just so happens to transit via point B..
you did NOT buy transport from B to C or C to B, or any other combination therefore-- only A to C..
While one can argue the economics to the airline in terms of not traveling-- and thus effectively forfeiting-- one leg of a ticket, the reality is that your ticket was priced based on the MARKET forces, airline competition, demand between the two points, etc for travel from City A to City C.. So the fact that it just so happens to transit a more favorable point for you is immaterial.. and remember in most cases, airlines do have latitude to change schedules which includes changing your routing as well as the addition or elimination of any transit point(s).
... and by you electing to skip one leg and try to board at the connection City B --- without airline approval-- you are effectively changing the terms of the contract you both agreed to; that being transport from City A to City C.. and as such, the airline would then be correct to impose their contractual rights which are commonly to deny all down-line travel and/or re-pricing your entire ticket to reflect travel from City B (not A) to City C.
Edited: 12 February 2012, 13:16