Technically For USB, you should typically draw no more than 100mA, unless you're enumerating the device and specifically asking to draw higher current. According to the spec, you can enumerate yourself and request up to 500mA in USB 2.0 spec. However, these limitations are rarely enforced physically, meaning that usually you will be able to draw until some circuitry in the PC limits you. So, in general, less than 500mA is safe.
The voltage regulator you chose is an older regulator that has very little short circuit protection, reverse voltage protection, overheating protection, etc. I suggest you select a more modern part that can do what you need and includes all of these things. Note that the microcontroller will likely need direct connection to the USB VCC.
Selecting the 3.3V regulator, you need to take into account primarily output current and dissipation (Aside from the obvious fixed voltage output and a Vin that includes 5V), although for 500mA it's not that big of a concern.
Your circuit needs a fuse as well. Select a fast acting fuse.
Also, I didn't see the typical ferrite bead and decoupling capacitors that are usually added. Your microcontroller likely needs direct connection to 5V bus to be able to operate with USB, so check the reference circuit for the part you're using.
Finally, if you're that worried about your Mac at first, power the board using some AC to 5V converter like the ones used for charging phones. Measure the current and make sure that the it's ok. Also, always measure whether there's a short circuit between the USB Bus VCC and GND before connecting it to anything.
The ARM processor you're using will be much less than the limit. Look at its datasheet on page 1013, it tells you that at 80MHz fully running it is 90mA. Add the extra stuff and GPIOs to it and USB is plenty.
Best Answer
Overvoltage isn't so much the issue as over-current. The host supplies the voltage and you can reasonably assume that it's not suddenly going to supply more than 5V.
The two most important failure modes to protect against are:
if your device is connected to anything else with a power supply of its own, that supply must not feed back into the USB port.
if there is a short circuit or overcurrent in the device, it must not draw more power than is reasonable for a USB device. (normally 500mA, although technically you have to negotiate the use of more than 100mA, and some standalone chargers may give you up to 2A)
A standard means of protecting against overcurrent damage is a polyfuse (resettable fuse). Current limiting in normal use can be done by one of many power controller ICs, which are also useful for supplying other voltages, charging batteries, identifying when a high-current charger is plugged in, etc.