The way I read news reports on the Gmail proposal was that the mail server would scan each email for certain words and then include a banner ad in the email based on the content of the email. What wasn't said was whether this would be included on incoming mail to the account holder, their outgoing mail or both. Then again the reports could also be wrong but I based my decision on the former just to be cautious. If the reports I read are wrong then I'd remove the block.
At the moment though, it really annoys me that the 'Net has become so reliant on advertising. I am not against people making money from it, the 'Net is a legitimate business tool, but in the right circumstances. I'm a freelance photographer and do plan on setting up a portal to sell photos one day, when I can be bothered getting one of the banks to give me a merchant account so I can take realtime credit card payments. Advertising wouldn't be so bad if it was more tasteful, most internet advertising pertains to penis enlargements and fake university degrees, neither of which are much use since the average bloke is built for the average sheila and I am quite sure that prosective employers can tell the difference between a degree from a legitimate university and a non-existant one.