>>I suppose I also don't know if Linux is one of the
>>systems that supports unloading modules.
> 
> 
> Linux supports this.
> Note:
>   Smalltalk unloadModule: 'LargeIntegers'
This is expected to work on all currently supported platforms. I've not 
yet come across any that cannot handle it.

>>A final issue is whether one
>>can unload a builtin module at all.
Same thing; except that it merely 'unlinks' the plugin from the internal 
list of active plugins. As soon as you invoke a method that refers to 
it, the plugin will be reloaded. NOTE however that (unless anyone has 
removed this) the plugin lookup starts by looking for an EXTERNAL plugin 
and thus any found external plugin will override the internal one. This 
for example means that it is trivial to override the facilities of the 
SecurityPlugin merely by implementing one that does the appropriate null 
actions.

tim