This question might be old, but I think it's very very important to add something to this.
Not only does the USB spec prohibit back-driving Vbus, but you will destroy your computer if you try this! This is not merely theoretical - someone destroyed their Macbook Pro by back-driving 5V into Vbus. It was deemed irreparable by Apple, requiring a new logic board. I imagine many other PCs can be destroyed this way. The damage was not confined to the USB port - the entire computer was ruined.
The number of errors I see in "hobbyist/maker" projects blows my mind sometimes. These people shouldn't be designing things to connect to expensive equipment like a computer at all. It seems lots of people just paste circuits around the community without ever taking the time to read spec documents when designing something. No-name Chinese manufacturers are also guilty of this. I can't stress the importance of complying with all the necessary specifications when you implement an interface. It's even possible that you could be legally liable if you publish a design that someone builds and causes damage.
So it is absolutely essential that you have a schottky diode in series with Vbus, if not a completely different design. Depending on the regulator used, it's not always necessary to protect that against back-drive - but always check the datasheet carefully.
Back-drive can occur in another way - simply by putting too much capacitance on Vbus. The USB spec requires no more than (IIRC) 10uF. Any more than that and you will back-drive too much energy into Vbus when the computer is turned off (not to mention the inrush energy being too large when powered up).
The more usual way to handle USB power is to use an LDO to bring it down to 3.3V and run your circuit from that. That also eliminates the problem of the potentially large tolerance of Vbus.
My personal feeling is that if an error this grave is present in the design, there are probably other very serious errors too. If possible, I would be inclined to source from another manufacturer/designer. It might seem like an unnecessary expense if you've already bought the board, but how much is the equipment the board will be connected to worth? What are the financial implications if those devices suffer damage as a result of an improper design? You might be able to salvage the board by doing some modifications to it, but you'll need to invest some time doing spec/datasheet reading first.
If the design uses a USB-to-serial conversion chip, there are some perils involved. Ideally the entire device wants to be powered from Vbus to eliminate issues - but make sure there is no possibility of back-drive by another device attached to your board such as a motor which might generate back-EMF or a serial data line which is pulled up to an external supply. If you're powering from a local supply, then you'll need to make sure the USB-to-serial chip can read Vbus to determine whether to drive D+/D-. One way to do this is to power the USB-to-serial chip from Vbus and the rest of your circuit from a separate supply. But now you have a new problem - latchup. There is the possibility that you might be driving logic signals into the rest of your circuit which is powered down, or the rest of your circuit drives logic signals into the USB-to-serial chip when it's powered down. Both can be catastrophic if you exceed the maximum clamping current (which may be listed in the datasheets of the relevant chips - or it may not). A series resistor can sometimes be used to limit the current, but never assume anything about the clamping current rating. Some devices can tolerate 10s of mA, others can only tolerate a few 100s of uA. This is just one of the reasons why multiple-supply designs can be a big headache and a real trap for those unaware of such issues. Life is so much easier if you can be certain there's only one power supply involved - and in the case of a USB device, that would be the computer's power supply. These days with low power MCUs and FPGAs, it's probably simplest to power everything 3.3/5V from Vbus and only use an external supply for things like motors. The transistors which drive the motors will provide the necessary isolation between the two supply domains. If using motor driver ICs, you need to check carefully to make sure the logic and high voltage parts can be powered independently with no sequencing requirements.