"via the Micro-USB", to be more precise.

Following with some reactions in the N900 Specifications thread:

There is no doubt that most users (even most Maemo target users) would prefer saving cables and having one single standard interface for charging over USB OTG.

Note that a separate charging port doesn't only mean more cables, but also more volume and complexity in the design of the device. If you think "it's only a small slot!" is because you (like me) are not really into engineering mobile hardware.

You can't connect a camera in host mode, but the N900 comes with a good camera. The use case is still relevant but is not as heavy as in previous Maemo devices.

You can't connect a hard drive in host mode, but the N900 comes with a big internal memory expandable with a MicroSD card. The use case is still relevant but is not as heavy as in previous Maemo devices.

USB OTG is in our radar for future releases, yes. Too soon to make an official commitment though.