Disclaimer: IANAL

Short answers:

• yes, you can use plupload without open sourcing your code
• no, minifying is not excluded by the license

Rationale based on my understanding to the GNU GPL 2.0 license is this:

1. as long as your code is not a derivative work of the GPL'ed work, you are fine. In this case, your code does not extend, modify or otherwise depend on plupload other than using it over well published interfaces, so it can hardly be argued to derive form it.
2. minifying the GPL'ed work is fine, as the minified version is really a binary (executable) version of the source code. This is permitted.
3. you have to offer to provide the GPL'ed code, that is plupload, on request to anyone using your website.

BTW: To be on the safe side, you may want to consider buying one of their commercial license. And no, I am not in any way affiliated to plupload.

Disclaimer: IANAL

Short answers:

• yes, you can use plupload without open sourcing your code
• no, minifying is not excluded by the license

Rationale based on my understanding to the GNU GPL 2.0 license is this:

1. as long as your code is not a derivative work of the GPL'ed work, you are fine. In this case, your code does not extend, modify or otherwise depend on plupload other than using it over well published interfaces, so it can hardly be argued to derive form it. Please note this interpretation is debated and untested in court (as per comment by @apsillers).
2. minifying the GPL'ed work is fine, as the minified version is really a binary (executable) version of the source code. This is permitted.
3. you have to offer to provide the GPL'ed code, that is plupload, on request to anyone using your website.

BTW: To be on the safe side, you may want to consider buying one of their commercial license. And no, I am not in any way affiliated to plupload.

** Updates

• noted emphasis on GPL 2.0
• added link to further information on derivate work interpretation