This is not an operating system feature, but mostly your USB controller: to make it work a part of the controller that controls power of USB port must work even when laptop is mostly powered down, and also it's voltage converter (Li-Ion battery provides approx. 7-14 V, and chipset requires at least 3.3V and 5V) must provide a reasonable current to charge something (1A is max for one port by USB specification) while scattering heat (remember, fan doesn't work).
So this is a feature that requires much more hardware support than software, and therefore it is present in a few devices. For example, my Toshiba NB200 (Intel ICH7 chipset) has one USB port out of 3 that's powered while it is suspended, and that feature is enabled in BIOS: I found no settings in XUbuntu that may affect it, through there was some Windows XP utility IIRC.
To check if your laptop has this feature you can:
(Most easy) Check it's case and try to find a small 'lightning' mark near USB ports. There is one on mine, but I don't know if this is a de-facto standard.
(The obvoius) As this feature is relatively rare vendor probably wrote info about it several times: on the original box, in user manual or on some stickers on laptop itself.
As this is mostly chipset feature, you can determine yours and check chipset vendor's documentation on website.