One of the odd challenges I encountered with getting Inkscape working on Sebastian were the fonts. The fonts showed up in the usual slew of non-X11 applications but refused to show up in Inkscape. I admit to not have tried the Gimp in this state although I assume this is a fonts-in-X11-apps issue and not specifically Inkscape and thus the Gimp would have had the same problems.
A bit of googling resulted in this forum post that solved the problem nicely.
Macs present both a system-specific (under /Library) and user-specific (under [User]/Library) view of several system resources, such as Fonts. The concept being that a specific user can install a resource under their login ID ([User]/Library/Fonts) and that resource is only available for that user’s login ID. Logging in as another user would not expose those resources. The end-user helpfully sees a superset of both the System and User resources exposed to them in the interface.
If you want a resource made available to all users on the system you place them in the System resources instead of the User resources and in order to install a System resource a privilege escalation is required. Like any good OS this helps limit the damage that any given user can inflict to the system.
Extrapolating from my geek knowledge, it appears the core of the issue is how X11 interacts with user resources — or rather how it doesn’t interact with them. My suspicion is that the X11 subsystem only pulls the System fonts, and not the superset of the System and User fonts, which is the reason why moving fonts from the User resources to the System resources caused them to show up in Inkscape.
Next up on my Mac OS X to-investigate list: making printer defaults stick in X11 apps.