I never saw any in fact. Do you know of any. I am just interested to see where they fit.
I never saw one for real, only in pictures, but they do exist and connect to the A500's expansio port. Which makes connecting your GVP HD8+ impossible... :-)
Ok, I understand now - the null modem connects to the hub and the hub connects to the ADSL modem right? or?
Right. But even then it is not guaranteed to work: even if you can find a hub with an RS232 port, you should check if that particular hub has a connection to guide the signal from RS232 to the RJ45 connectors. Many hubs with an RS232 connector just use it to edit the settings of the hub itself, without being capable of transferring data to and from the RJ45 connectors.
I do not know if i'll manage to do it. However I have my laptop to browse the internet. All I want is the see it work and maybe send some email or two ... with the title; I am sending this from my A500+ 
E-mail will work nicely (albeit a little slow on a 68k), as does FTP, IRC and some other internetstuff. To be short: browsing the web sucks on any Amiga, the rest is ok.
If you have trouble locating an adequate hub (which would not be strange), connecting your Amiga to a pc with a null modem cable and use the pc as a sort of router/server is much easier to do and gives the same results in the end.
My A1200, 2000 & 3000 are all connected to my LAN and this works quite nicely. All of them use a 'real' ethernet card, which is very easy to set up.