Today I found that ports 1-10000 were being forwarded on an Airport Express that I as well as a fair number of other people share. This was setup by another user. There are multiple machines and devices on the network, OS X and Windows, and I was wondering what having those ports forwarded means to the various computers on the network and if there are any potential issues for this situation.
Functionally, it would be as if the machine to which the ports were forwarded was directly connected to the Internet. (At least for those ports.)
To the other machines on the network, the forwarding itself wouldn't be much of an issue, but if the forwarding target gets compromised, it could pose a threat to other machines on the network.
The first thing you should find out is what machine the ports were being forwarded to. If that IP is not in use, then the forwarding should not be a significant issue. If the target is patched with no services exposed to the Internet, then having ports forwarded is not likely to be a significant issue. (Most clients that are compromised are done in a way that NAT won't protect you against anyway.)
Forwarding simply removes the protection of network address translation (NAT). This means for most effective purposes that the computer which is being forwarded too has about the same level of access you'd have being on a local LAN with it via the internet.
It doesn't mean it's inherently insecure, in fact it has identical security to what it has against other LAN users on the network. A key difference is though now instead of however many malicious users you have on the LAN looking (perhaps 1?) you have thousands on the internet checking it out.