Emmissions Workshop, sound familiar?

Hi all,

I can't thank this group enough for the number of times I've saved $ by googling my passat problems and landing here. I'm currently following the good advice given in THIS (http://www.automotiveforums.com/vbulletin/showthread.php?t=359851) thread for the all too familiar check engine light/emmissions workshop message.

I'm starting with cleaning the air screen, replacing the air filter, and cleaning the MAF. I'm handy, but I know nearly nothing about cars. Little did I know that I would be forced into avoiding expensive repairs by doing my own maintenance when I bought the thing, but I digress.

Here's my question (finally). When I opened up the airbox and pulled the filter, I noticed a little round plasic spring loaded valve looking thing at the bottom. It has a crappy foam disk shapped gasket that has turned to dust. Do I need to replace that? If so, what is it called? And just out of idle curiosity what does it do?

Any think else I should know about this fine kettle of fish I've gotten myself into?

Thanks in advance for any help.

Edit: 2000 Passat 2.8 V6 GLS, in case you were wondering.