Not necessarily. To be more precise, USB 2 specifies power in up to 5 steps in 100mA increments. Every device must start as a low power device, consuming one power unit. That is up to 100mA, not necessarily 100mA. Only after a successful enumeration and negotiation with the host for more power is the device allowed to switch to consuming more power unit, up to a maximum of 500mA. At least such is the theory. In practice, this is simplified to "low power" (up to 100mA) and "high power" (up to 500mA).

I have seen compliant devices that take exactly 0mA in the "low power" mode. For example the N900 itself, when connected to the PC. I have also seen non-compliant devices that take more than 500mA before they even connect properly. For example el-cheapo USB HDD enclosures.

Your hub is unlikely to consume the full 100mA. It is probably a low-power device and is allowed up to that much by the standard, but it is more likely to take only about 20mA, leaving a bit more for any attached devices.

When in doubt, invest in one of these gizmos and measure it yourself.