Okay, I'm relatively clueless about linux, so please bear with me.

I have a linode account. What procedure must I got through in order to enable:

echo hello world | mail johnny@dewey.com

I also have a few domains at godaddy, would I have to change some MX records etc to my linode host's IP address? It's running ubuntu 9.04 if that changes anything.

ok well first you should upgrade to ubuntu 10.04, 9.04's support ended in october, see http://library.linode.com/troubleshooti ... 0.04-lucid for instructions

Then you need to install a mail server, either
http://library.linode.com/email/exim/se ... 0.04-lucid
or
http://library.linode.com/email/postfix ... 0.04-lucid

Then mail will work

_________________
Paid support
How to ask for help
1. Give details of your problem
2. Post any errors
3. Post relevant logs.
4. Don't hide details i.e. your domain, it just makes things harder
5. Be polite or you'll be eaten by a grue

Will sending mail as-is, say, to my gmail account possibly mark my IP as a spammer address though?

nah, especially if you add a SPF record, you need to add a TXT record for your domains DNS with, here is a good wizard http://old.openspf.org/wizard.html

_________________
Paid support
How to ask for help
1. Give details of your problem
2. Post any errors
3. Post relevant logs.
4. Don't hide details i.e. your domain, it just makes things harder
5. Be polite or you'll be eaten by a grue