Copy/pasting from the original bug report I posted on fluxbox's bug tracker (http://sourceforge.net/tracker/?func=detail&aid=3208974&group_id=35398&atid=413960):

The GUI for the virtual currency program Bitcoin (http://bitcoin.org) does not show up at all. There is just a very small blinking phenomenon during less than one second at program launch in the task bar (using gnome-panel). This program works fine under gnome and there is also another report of the same thing happening with Fluxbox on the Bitcoin forum.
Bitcoin still shows up in 'top' and seems to be running normally even though nothing shows up.
This could be an incompatibility with wxwidgets.

Versions tested: Fluxbox 1.3.1 and Fluxbox 1.1.1 (Ubuntu Maverick), Bitcoin 0.3.19 and 0.3.20

I got this answer:

Date: 2011-03-15 09:55:42 UTC
Sender: akir

i would point to bitcoins code and that they should fix their application:
https://github.com/bitcoin/bitcoin/blob/master/ui.cpp#L2768