Ummm -- if you're a business entity you might want to consider whether you're operation on a cash or accrual basis.
Also, I think the previous post is incorrect even if you're on a cash basis. I think that the date of the check is operative, not the date you receive it. For example, if you're an employee and your company cuts you a check on December 31 and mails it to you, and you receive it on Jan 2 and cash it in January, I'm pretty sure it is previous year earnings. I'm not sure if the same rule would apply to a business entity operating on a cash basis.
To state that point differently, while earnings "received" not "earned" is the question for a cash basis taxpayer, I think it's deemed to be "received" on the check issuance date, not the date you get it in the mail nor the date you cash it.
For business entities operating on an accrual basis, I'm 99.9% sure it would count as 2004 earnings.
P.S. As an individual, you are always on a cash basis.
P.P.S. <disclaimer>I am not a tax advisor, nor do I play one on TV. Consult somebody who actually knows ;-)</disclaimer>