Android 2.1, HTC Hero EU and WIFI configuration issues
From what I've read this is more than just an issue with the HTC Hero and seemingly still hasn't been fixed completely(scroll to bottom of the page).  However I did find several suggestions:


One of the comments claimed he "choose 'up to 54mbps' for hidden SSID on my netgear router" and it worked - maybe this will work for you.
I also read (and confirmed) that if you are using a router with b/g/n - you need to switch it to just b/g mode for it work with a HTC Hero.  I have not confirmed this with a hidden SSID (confirmed with a broadcasted SSID) but its a suggestion to try if you don't mind using only b/g mode. (I'm pretty sure this is the same as suggestion 1 but just in case he meant something else I provided this)
You could also try HiddenSSID Enabler.  I have not tried this so I can't confirm but the comments on the app seem to indicate it works fine.


Hope one of these suggestions works for you.