I too have a cable modem, and over the past year or so there have been at least a dozen unwelcomed attempts to enter my machine. You can't rely on your ISP for any help with this.
I get daily scans/attempts on my cable line and my DSL line. You are 100% correct. Both my ISPs really don't care.
I would suggest that you disable all services you don't absolutely need, such as finger, daytime etc. I also suggest disabling ftp, telnet, and rlogin. If you need to login to your machine from a remote location, use the much more secure SSH. You can comment out the services you don't need in the files /etc/rc.config and /etc/inetd.
You can get SSH at: ftp://ftp.cs.hut.fi/pub/ssh Also, take some time and read the Firewall and Security HOW-TOs: http://metalab.unc.edu/LDP/HOWTO/HOWTO-INDEX.html M