Do ISPs really care about whether people run servers on residential accounts
Depends on who you ask .. but generally, as long as you pay your bill and you don't make them do paperwork on your behalf (eg: DMCA crapola), they ignore it.
do they scan ports? If so, how often?
Again .. depends. Back when I had ComCrap (Comcast), I'd get hits on tcp/21 fairly often. Run your ftpd on some obscure port (better yet with starttls) and you'll be fine.
Will they be able to decrypt the data from a middle node?
Not in this lifetime.
Is it worth also running a public web\ftp server (on a different port than 80\21)?
Always .. they don't have the patience to scan every port .. avoid the "well-known" ones and you'll be fine.
If they find out, will it be a warning letter or termination?
Usually you get a warning first, unless they get a DMCA or some-such on your behalf .. then you generally get a 1-year ban from that company.
On the plus side, getting canceled by them gets you out of your contract agreements. Play your cards right and keep mis-spelling your name when you sign-up, and you can switch between cable and DSL forever.
Cheers, Michael Holstein CISSP GCIA Cleveland State University