I have been using MacFreelance for this purpose, and find it pretty useful. The interface isn't nearly as intuitive as iCash, but it makes solid Invoices once you figure it out. You can use it for keeping track of time, or for fixed rate items, and it clearly shows what invoices are outstanding.
I personally like keeping it separate from the accounting software as it by itself is a fairly complex process. However, it could be useful to have them connected.