Oh, sure, it exists in your mind and you're aware someone owes it to you, but until you have it -- it really doesn't exist.
I was doing my taxes the other day and it asked if I wanted to keep records by accrual or cash. I chose cash. Accrual is for all the jobs that you billed during the year, even if you didn'f get checks from them all in that same year. I chose cash, which is taxing myself only on the checks I received, even if it was work from late 2005. I just choose to keep all the money real, I guess.
Good luck with tax season, all you writers!