Microsoft and Novell today announced an expansion of their 2006 interoperability agreement, with Microsoft pledging to purchase up to $100 million in certificates that customers can redeem for support. The investment is on top of the $240 million Microsoft initially agreed to spend as part of a five-year business and technology deal with Novell that also included intellectual property rights protection.

While open-source advocates have been critical of Novell’s patent protection agreement with Microsoft, companies tasked with running mixed-operating system environments have actually welcomed it. Novell says it has already invoiced more than $157 million in certificate revenue in 18 months, so this latest cash injection is undoubtedly good news for the Linux vendor.