What would happen to my mobile bitcoin wallet if something happens to my phone? If it breaks, or I want to replace it or it gets stolen?
You lose your money.
To mitigate this encrypt your wallet and do regular backups to several other places. I think wallets will have the backup functionality built-in soon, but it is not here yet.
If you lose your wallet, your bitcoins will be lost, unless you created a backup.
When you just want to switch to a new phone, you can either transfer the wallet to the new phone or sent the remaining bitcoins to another wallet you control.
Any and all bitcoin wallets should be backuped.
In case you have an online wallet (like coinbase) you won't have lost your wallet and you will be able to access your account from your web broser. In coinbase's case you can set it to allow payments without password, require password at startup or require it when a payment is to be made. Depending on what security you set your money can be used by the one who finds your phone or not.
I think it is a good idea to have a couple of wallets, the one on the cell phone should be one with enough money to buy what you may need as daily or monthly expenses, but it SHOULDN'T have ALL your coins. Keep those in a safe place, like a paper wallet or encrypted in the cloud.