1) They reside in the database (at least references to the plugin are in the database) -- I believe.
2) You can try to uninstall. This might not work with the plugin not available. Therefore you may want to upload the original plugin even if you don't want to use it.
3) You can upload the plugin, uninstall / install again and see which ones work with 2.0 and which ones don't.

*edit* yes, the class Plugins puts basic info in the database and you can tick the red uninstall button to delete this info (uninstall the plugin). There is no need to have a copy of the original plugin available.

Good luck

Thanks Afwas.... I'll give answer 2 and 3 a spin later on.

Thanks.

Plugins is what's keeping me from updating my production blog... haven't tested it on my test server yet, still updating some skins to 2.0