I'm a big fan of the tagging concept: being able to create new ones on the fly ... etc. But I think that implementing it outside of the current category system would be a mistake.
Do me a favor, and before you try implementing tags, have a look at XMP ( http://www.adobe.com/products/xmp/ | http://en.wikipedia.org/wiki/Extensible_Metadata_Platform ) and in particular, try playing with tagging in Windows Vista Photo Gallery.
They've done some cool things with tags and they have the concept of nested tags. You can start typing the deepest level and have the tagging tool recognize it, so ... as I'm typing "Mik" it recognizes it as the beginning the tag: "Photos\Family\Girls\Mikayla" ... And when applied, it means that this thing I just tagged matches "Mikayla" but also "Girls" and "Family" and "Photos" ...
I guess what I'm getting at is two things:
There's no reason tags can't be nested like: "Photos\Family\Wife", or "Photos\Places\New York\Statue Of Liberty" -- which means that if you're willing to just stick to a short list of them, they'll work exactly the way categories do.
If you develop nested, dynamic tags, instead of relatively static categories, you also end up with tools which will make most of the other category-related ideas I'm seeing easy or automatic (like moving posts between categories, batch category editing, multiple-category search, per-user category navigation, etc).
Searching for "Blogging" + "Ideas" should be able to turn up all the posts in "Blogging\WordPess\Ideas" along with all the ones in "Blogging\Ideas" or ones that are tagged with both "Development\Ideas" and "Blogging" ... and you really ought to be able to link to that, like mysite.com\categories\Blogging+Ideas\