If I become Power of Attorney for my aging parents, am I responsible for their future expenses?

If you have their POA only, no, you would not be responsible for any of their assets, investments, accounts, etc. The exception is if you are also named as a co-owner on accounts such as checking accounts or if you are also on credit card accounts.
The POA only gives you the ability to make financial decisions for someone who has become incapacitated.

The answer above is correct. You are managing their affairs, and have a duty to be financially responsible on their behalf, etc. but not pay expenses out of your own funds or take on liability personally for anything of theirs (if, as stated above, you don't get into joint ownership or anything). Also, be careful of signing things on their behalf (i.e. a nursing home, assisted living contract, contract for services, etc.)-you need to sign as their POA not as yourself (i.e. "John Smith as POA for Loretta Smith"), in which case you could be held personally responsible.

Also, most POAs are effective immediately (not just based on incapacity, unlike medical POAs/healthcare surrogates which usually are only effective then)...though most people don't use them until the person has difficulties managing or is unable. The language also must be "durable", meaning that the POA survives incapacity (other POAs are sometimes used for limited cases, i.e. selling property). All of this brings up the importance of getting good legal advice...forms can be ok for some things, but a good estate planning/elder lawyer can answer all these ?s, customize documents, keep you up to date on any law changes, etc.

Lilliput, my mother made me co-owner of her personal checking, money market and CD's with right of survivorship before she made me durable POA. What are my responsibilities there? Once she got in the nursing home, she made me joint owner with right of survivorship of her personal securities which before had been in her name only?

Crowe: from my limited experience with these matters and from reading about other forum member's experiences, I believe that all the assets, where you are named "co-owner" will revert to you upon her passing.
Did she also create a will or living trust? If a will or trust states otherwise, I am not sure which would take precedence...this would be a good time to contact an impartial attorney to evaluate her documents. (I am hoping one of our experts chimes in on this issue.)
The POA is a document that allows you to help your Mom carry out her wishes if she should become incpacitated in any way. In essence, all her income, assets, and properties are hers to do with as she pleases until her passing.

Lilliput, my mother created a will and named me her sole beneficiary.

My mother made me her POA and also put me as co-owner on her car and home. She lets unlicensed people drive her car. Am I personally responsible for damages caused by those unlicensed drivers?

Are both your and your mom's names on the car insurance policy? Is your mother competent to handle her business in a business like manner and has a doctor evaluated her competency? You and your mother both own the car and are responsible for how it gets used or misused.

My sister has POA for all my dads affairs, can she stop me from asking doctors and nurses what is going on with my dad?

My mum is in a nursing home, I am POA and her creditors are harrassing me about her bills,. She gets Medicare, all of it goes to the nursing home,with nothing left over to pay any of her creditors. The creditors wants me to fax a copy of the POA. If I were to do that ,will they hold me responsible for paying her bills.?

The creditors want some validation that you have durable POA. Sending them a copy confirms that but does not make you responsible to pay your mother's bills from your own money. You might need to find a valid debt agency to help you deal with these creditors. Don't let them bully you into thinking you have to personally pay them out of your own pocket.