Perhaps instead of using the ieee80211 stack + r8187 driver you could try the mac80211 stack + rtl8187 driver? Provided of course the mac80211 stack is available/compatible with your kernel. Maemo5 has a much newer kernel so it should be do-able.

In my experience both drivers work well with this card, though the r8187 driver works slightly better in monitor/injection mode than the rtl8187 and OTOH the rtl8187 works slightly better in managed/web-browsing mode than the r8187. Yet both are perfectly capable of both managed and monitor mode.

Anyway give the mac80211+rtl8187 route a shot if possible.

The unknown symbols error usually means either it wasn't compiled cleanly or that the driver source needs some moderate (to major) changes in the code to work properly with your kernel.

It's good to hear that usb host-mode is finally working on the N900 (although it's possible that it's the culprit). Oh and you are providing the card adequate external power, yes?