The new tag system includes an importer for those using the Ultimate Tag Warrior plugin, and importers for a couple of other popular plugins are in the works. There is also a conversion process available for those who would like to convert their existing categories to tags. Due to the flexibility of the new tag system (taxonomy system is more accurate), I expect we will see an explosion of new tagging plugins.
There have been reports of compatibility issues with a few plugins due to the changes to the category system (plugins that only used the API functions should be okay, but those that used direct SQL queries may break). Also, some file moves in the admin area have affected a few other plugins. Please report problems on Trac, and document compatible plugins on the Codex.