You're not alone. Due to the economy, MANY people's workloads have increased (although in your case it doesn't seem to have anything to do with staff being let go). More work can be a good thing (regarding your career aspirations) or a bad thing, depending on the mentality of who you work for. Things also depend on if you're working for a moms & pops boutique or a large retailer.
The good news is...if you're capable of taking on more responsibilities, then do so. That will give you a great opportunity to show what you're made off. Picking up the slack is often times appreciated by management...they'll see you as a "company man/woman." You'll quickly go up a notch in the store's food chain. When the economy picks up, they'll keep you in mind...they'll reward you (i.e. raise, promotion, etc...).
Lets say you're now working 45 hours/week as opposed to 40. That extra hour per day will get you more exposure/recognition. You'll be more visible to the higher ups. The next time they need a position filled...your name might be on the top of that list. Just make sure to document any "extra" tasks that you take on...as to receive credit for them.
You'll also have some cleavage...I mean leverage, sorry

(I can't get that HELL thread out of my head

). Ask/demand something...a raise or increased benefits

. If you end up leaving, then productivity will suffer...that will make your manager look bad (she's aware of this, I'm sure). Hence, she might be more willing to accommodate your requests.
On the other hand...the bad news is...your bosses might expect you to take on more work, with a smile

. "
There are millions of people out of work...and you're complaining???" If they get too comfortable with the thought of you being their work horse...then they might go as far as to fire a staff member (more fruitless work for you)...
"Well...Christine will take over their responsibilities and we'll have one less employee to pay."
Also, more work doesn't necessarily mean more productivity. If the extra unappreciated hours are cutting into your social time...you'll start feeling it, and it'll show! You'll be exhausted, you'll have more stress, you might even fall into depression. Not only is this unhealthy, it'll also end up getting you fired anyway (because you wont get anything done).
Its good that you've addressed this with your supervisor. Perhaps touching base with Human Resources might be something to consider as well

. Hell, maybe even call an extra office meeting once a week. Take the initiative with a "
lets work as a team" speech. You'll be surprised...it might not fall on deaf ears (as the team leader, you
must demand that it doesn't).
Your boss is aware that you have your own work/deadlines to meet...non the less, keep reminding her of it. If she does nothing about it, then go as far as to refuse to take on the extra work (within reason). Eventually management will notice that productivity has decreased. Perhaps they are so used to everything getting done...that they might not realize that
you're the wheel behind that machine

.
At the end of the day you must respect yourself...you need to put a value on your efforts and not allow to be taken advantage of. If nothing changes...start looking for a new job. If your work history shows you to be a consistently hard worker (which it looks like you are), then you should have an easier time finding more work.
Of course, you can also just stay quiet and say "yes man, yes sir." However, for the above reasons, I don't think that's a good idea.
Either way, good luck.