True, it takes 3 days to settle,but with Fidelity, and I don't see why it would be any different with any of the large firms, I can sell stk A, and use the cash to buy stk B immediately...do it all the time.

When I sell a mutual fund, I can't make a stk buy until the next day. I can also sell a MF and use the proceeds to buy another MF, but the transaction takes 1 or 2 business days to complete, depending on the fund.