[Q] Does anybody know if the bigger convenience base thing gives you any more
than the mobile port expander?

[A] In addition to the spot for recharging a second battery pack as mentioned
in other responses, the Convenience Base offers the following ports: VGA, PS/2
Mouse, Keyboard (PS/2 style), Serial, Parallel.

The serial and parallel connectors on the Convenience Base are pass-through
connectors. The S & P ports on the back of the Aero are passed through the
expansion port. This means that you could leave a printer and an external modem
(or other serial/parallel devices) attached to the convenience base. Just slide
the Aero into it you're all set -- it looks like it'd be a lot more convenient
than finding your printer cable and serial cable and plugging it in every time
you wanted to print or transfer files to/from your desktop machine. The base
comes with its own AC adapter. It will recharge a spare battery and the one in
the Aero (however, not simultaneously: it will charge the Aero battery fully
before it charges the spare).