If someone gets an open source template and then puts a lot of work into tweaking and changing it, can they then claim copyright on the finished work? Or does it remain open source because the base of the template was that way?

Generally the GPL will tell you that you are not permitted to use any component of the software that is provided under a GPL, in a proprietary piece of software. If you use GPL as it's basis, you HAVE to keep it GPL.

What's more depending on what the software is you are tweaking, you may be required to send the changes back to the person who owns the GPL for that item. That way they get to choose all the best tweaks and additions and include them in the next version, and refuse the ones they do not consider good.

This is to reduce the risk of splintering, as happened with Unix. Unix was such a wonderful OS, but everyone went off developing it on their own and they ended up with such vastly different versions that what ran on one version wouldn't run on another. So much so that it becam useless as an OS, and this ultimately was it's downfall.

So play nice! If you change it, pay back the favour that was having the GPL template or piece of software in the first place!

Normally when you tweak a template you MUST leave the original copyright. You can place a second copyright indicating all editing, changing, etc is your own copyright. I've done this quite a few times and allowed others to do the same. Its a bit cluttered looking depending on the layout, but its only fair to give credit to the original coder, even if they wouldn't recognize their coding after you are done.

Okay, so playing around with the code is cool as long as I still give credit to the original coder. I'm thinking of doing something similar to this:

Template by so-and-so, tweaked by me

Does that satisfy the requirements?

Each template will have its own requirements so I can't give you such a statement without knowing the exact template and what site it may have come from. Most of them will give their usage terms in the body of the template to serve as dummy text.

Most of the ones I use, as well as any I've distrubuted require the original copyright link and text to remain unchanged. It says soemthing like "template provided by soandso" with soandso being a link to a personal website or the user account on a template site.

I guess you want to know if your template must be "free to use" as well. You need to carefully read the license of the specific template. There's no more to it than that really. I imagine you might be able to convince the copyright owner to cut you some slack, if he sees that you put a lot of creativity into your design and it barely resembles the original. You never know.

if you add the designer credits in you're web site it's free to use

Incorrect. GPL is a very specific licensing and is that way because of previous problems with Unix splintering. The GPL concept was developed in such a way as to stop the same happening to other things in future.

You have to read, carefully, the license that comes with the item that you wish to further develop as that will tell you all that you need to know, you cannot just make assumptions because that is where you will get into trouble.

It would depend on the licence under which the tempate was obtained. By the way which template are you talking about?