I suppose it depends on the nature of your business - if you're looking for e-mail leads for a mortgage service for example then it might be appropriate for users to land on a contact form straight away, with a quick summary of the services the company offers. If you're an e-commerce retailer where users want to browse products etc, then its probably more appropriate somewhere else.
I guess the questions I would ask are:
1) Is the e-mail form the goal of the website? 2) Does the user need more information about the company before contacting you?
If the answers are yes and no respectively, then a contact form on the home page would seem good. Otherwise I would recommend you use this space for providing other information or options.
Putting a "contact us mail form" (which the OP is asking about) on your first page is different from putting contact information on that - and every - page. IMHO, JamieBrown was on target in saying that it depends on the purpose of that page. Is sending the form the main action you want the visitor to perform? Then put it on the first page they see. OTOH, if the form is simply there in case an occasional visitor might need it, then putting it on a separate "contact us" page would make more sense.