Katy,
Here is what I know. First you would have to buy your home between April 8,2008 and July of 2009. Second, the credit isn't really a credit, it is loan. I know this is confusing. You would claim the credit on next years taxes if you buy this year. The credit will be credited to what you owe in income taxes. However, the credit is an interest free loan. The other part is that your first payment on it wouldn't be due until 2 years later.
I believe the note term is 10 years. If you sold your house before the 10 years, the government would take the money from the profit upon the sale of the home.

The tax credit is only for first time home buyers. It is for $7,500. Here's how it works when you file your taxes in 2009, you will receive a credit of $7,500. If you owe money the amount you owe will be subtracted from the $7,500. If you don't owe money or are receiving a refund the amount of your refund will increase by $7,500. Then beginning in 2010 (when you file your 2010 tax return) you will owe an additional $500 in tax each year until such time as the $7,500 is paid off. However, if you sell the house before the $7,500 is paid off, the balance will be subtracted from the proceeds of the sale.

Here's an example: You puchase your very first home this year. When you file your tax return in February 2009 you don't owe any taxes. But, because you purchased your first home in 2008 the government will send you a check for $7,500. When you file your 2010 tax return you will pay an additional $500 in taxes, and each year thereafter until the $7,500 is paid off. You sell the home in September 2012 for a profit of $10,000. However, you still owe $6,500 on the first time home buyer tax credit. Consequently, the $6,500 will be deducted from your $10,000 profit to pay the loan in full.

There are a few other nuances to the program. I recommend you visit http://www.federalhousingtaxcredit.com to get a full explanation of the program.

As for borrowing the money on a personal loan from the bank - most lenders will want to verify the funds for where your downpayment originated. When they find out it was from a loan from another financial institution - most lenders will not allow the transaction to proceed. They will be afraid that the bank who loaned you the money will attach a lien to your new home and that lien will have precedent over the mortgage. If possible, I would recommend doing an FHA mortgage and using downpayment assistance funds. But, be warned, downpayment assistance programs will not be allowed after October 1, 2008. So, time is of the essence.

You may find a bank that is willing to make a personal loan to you for that period of time. You do realize that the credit is in effect a no interest loan which you must pay back to the government over the next 15 years at $500/year? This is also a credit against taxes you may owe, so if you end up owing taxes, the credit is applied towards what you owe, it's not a $7500 check from the government necessarily.