Always always always always always always always always always cloak your affiliate links. I've lost count the number of email marketing messages I receive that have stealable affiliate links....
Clickbank in particular is easy to hijack...but if you use an autoforward for your affiliate link, you can cut down dramatically on lost commissions. You need to use the 'meta-refresh' tag -
http://en.wikipedia.org/wiki/META_refresh has an explanation of it.
Barbara