Some of the code requires knowing if a module has been loaded or not.

For instance, soon the forum module will need to ask

If ( google-mapping-module-installed ) {
google-mapping-module->ShowSomeMap
}

Hayden, can you please add this system?

a) Consult Sam to see about this thinking on it
b) Note that Andrew is working on related code that only loads modules if a _config.php exists See my top post at http://www.silverstripe.com/core-team-discussion/flat/1202/

Simplest solution here is to use class_exists, on the relevant class within that module.

For example:

if(class_exists('GoogleMapField')) {
$fields->addFieldToTab(new GoogleMapField());
} else {
$fields->addFieldToTab(new TextField());
}

now that we (and external developers) start building modules/plugins, we should think about setting up a basic plugin architecture with hooks into core functionality. some reasons:
- module developers are not dependent on core changes if the right hooks are existing (e.g. to add cmsfields to the standard page-object)
- API changes in module/core don't impact each other heavily
- module developers can release updates independently of the core release cycle
- the core stays cleaner and is easier to debug

we need to think on a broader level here, because we're laying the foundations for future module development. if we just write ad-hoc solutions (hacks?) into sapphire/cms, it just creates more problems on the long run.

matt had some thoughts on this as well, and it might fall in the scope of his GSOC project (mashups).