[ghshephard]

If you are going to specify a currency, write USD 10 (though spoken, it's 10 USD).

If the context is explicitly local (such as a local newspaper, menu), then $10 is sufficient in the United States.

[techdragon]

I used to do "10 USD" or "USD 10" until I got sick of hearing responses like

"USD 10 looks weird, why did you do that', or 'that on the pricing page looks funny, can you fix it up a bit'

It seems $ (or the equivalent currency symbol for other currencies) has a place in many peoples minds implying that the number it is next to is currency, and they seem to find it weird when things involving currency are 'written correctly' without the symbol that the numbers mean currency.