How could abuse in such as situation be prevented?
1) Have a PHP script that does the mailing, but only allow it to mail a static text thingey without customizable text
2) Have a PHP script that allows you to submit a message, but rather than sending it immediately have it queued somewhere for someone (=you) to confirm it being sent, to prevent abuse. Probably not too practical, though.
That said, personally I abolutely hate it when someone I know submitted my address through one of those 'forward to a friend' dealies. You always feel like they just handed over your address to some spambot. It just seems fishy why some random website would 'need' your email address... If anything it would make me suspicious of the site in question rather than 'greatful' for the 'exciting news'
Of course, your call. :-)