Ok I'm now sure that the Google Gmail service was not a 1 April joke from Google.
Why do they offer 1GB you ask? Well most users will only use a few 10MB's on short-term, and people interested to use this for warez will not be given a chance I think. Google will probably have developed a routine script or something like that which looks for 'strange behaviour' on their webservers.
With "strange behaviour" I mean things like users who upload an illigal program in packets (most emailservers only allow an incoming/outgoing e-mail to be a few MB maximum.). It's probably not too hard for a company like Google to make scripts that check for these things on a regular base.
This will be combined with an upload limit I think of maximum a few MB's for each email. A spam filter also lowers the amount of emails and thus each account needs less space.
One of the reasons I can think of why they are going to offer such a huge amount of space is to overwhelm users. A new e-mail service needs to be better than the competitors in orde to attract many users. Currently Hotmail for instance only allows 2MB, and for many users this is just not enough. Having to delete e-mail every week is boring and annoying. So there might be a chance that because of Google's respected brand and their new promising offer many users are going to transfer from Microsoft's Hotmail to Google's Gmail.
But why would Google be interested in giving millions of users the ability to have a 1GB account, are they crazy?
Well no, there is a very logical explanation for this. Each email will contain text advertisements, which are based on the content of each email. This system works exactly the same as Google Adwords/Adsense. So if this new concept turns out to work well then Google has just found a new goldmine.
I have been reading a few stories in which people complain about potential privacy issues. Well the contextual ads are based on a program from Google which tries to find out which are the most important keywords in the e-mail you just wrote. On the base of this it searches for advertisements which are related to the context of your e-mail. Thus nobody from Google actually reads your e-mails.