i too am using pound on ubuntu, and i was able to compile and install 2.1 with no issues ( after running apt-get install build-essential;
apt-get build-dep pound thanks adam!)

well, no issues other than the one i am about to explain.

i set it up to proxy for two servers, one is a windows2003 server running iis, the other is a linux based machine.

both are VMWare virtual machines that are actually running on the ubuntu box.

so here is the setup:
ubuntu 6.1 is running pound ( 192.168.2.20)
win2003 running iis ( 192.168.2.21)
LAMP host running mediawiki (192.168.2.22)
firewall on default gateway ( 192.168.2.1) forwarding port 80 to 192.168.2.20

the problem: the virtual hosts on the IIS box get error 503, while the linux based host displays just fine.

the IIS host displays pages just fine when port 80 is forwarded to it from the firewall.

here is the config (the tabs are probably going to buthcher it):

        ListenHTTP
            Address 192.168.2.20
            Port    80

            Service
                HeadRequire "Host: .*vh1.iishost.com.*"
                HeadRequire "Host: .*vh2.iishost.com.*"

                HeadDeny    "Host: .*vh1.lamphost.com.*"

                BackEnd
                    Address 192.168.2.21
                    Port    80
                End
            End

            Service
                HeadRequire "Host: .* vh1.lamphost.com.*"

                BackEnd
                    Address 192.168.2.22
                    Port    80
                End
            End
        End

i am sure i am missing something important in the config file, but i have no clue where to start.

to re-state the problem, i have an IIS host that serves pages fine with port 80 forwarded to it. when i use pound on my ubuntu host, and forward port 80 to it, i get the site hosted on the LAMPhost just fine, but a 503 error on the IIShost.
the problem is easy to get around for the time being, i just go back to forwarding port 80 to the IIShost.