If you are using the cash method of accounting, it's income in '05, and if you are using accrual accounting, it's income in '04.
If you don't know that type of accounting you are using, it is probably cash, as that is the most common and easily navigated method.
In cash accounting, money is counted in income when it becomes available for your use. This would be when you received the check, not when you sent the bill.
Snipes