We had to use a separate Posterous account due to the fact that Posterous' Autopost feature is account-wide, not site-specific. So since we wanted to connect the company Twitter account http://twitter.com/siyelo - it would meant both (my) personal blog entries (and Siyelo's) would autopost to Siyelo's Twitter - no dice!

So to those of you wanting to set up a Posterous blog for your company - use a separate Posterous account.