Amending 1040 too soon?

I e-filed my tax return on 3/12/11 and it was accepted the next day. After I filed, I realized that I left off a charitable contribution to United Way in the amount of $1100.00. It's an automatic payroll deduction and it completely slipped my mind when I was doing the return in TurboTax. I had a rather large refund coming which I received yesterday.

I want to amend my return to add this deduction and get a little more money back but I think I remember reading here one time that it's best not to file a 1040X too soon after filing the 1040 because the original return might not be in the system yet (or something like that). Since my refund was already paid out, does that mean that it's safe now to go ahead and e-file the

Thanks!