I don't know why the Netgear (a 5 port swith according to http://www.netgear.com/pdf_docs/fs605v2 ... pt2003.pdf
) would give out IP addresses, it's just a switch!
It's hard to picture your setup here in text. If you could draw a pciture somehow of how things are physically connected that would help.
It sounds like you saying your MAC essentially has two ethernet interfaces, 1 hardwired to the Linksys (or netgear) and the other to the Airport? And it's getting IP's from both sides?
If that is the case, if I were you, I would hardwire the ROKU to the same side of the MAC as the Linksys/netgear, not the Airport side.