Well, it might come as a shock to you, but most ISPs have been running on open source for most of their lives. Look at the Internet today, and you will see a bunch of
Linux and
FreeBSD servers staffed with
Apache,
MySQL,
Sendmail,
INN,
SQUID,
etc.
Closed source software solutions were, are and will be in minority. One reason for this is that ISPs usually have qualified people (yeah, yeah, I know), which are capable of reading documentation, tweaking configuration, and patching (yes, patching) their systems.
If you want to see the progress of open source, check out the average law office, or an accounting firm. Chances are that they have a Linux machine sitting somewhere in the corner doing file and printing sharing (Samba), and maybe some email. Desktops are not yet that often, but it will come eventually. There are many success stories today, but there are much more to come.
Leonid Mamtchenkov aka TVSET