On behalf of Canonical and the Ubuntu Technical Board, Scott James Remnant has issued a position statement surrounding Mono, the open-source project that provides a C# compiler for Linux (along with other operating systems) and other components for a free software .NET stack. However, the legality of Mono is frequently a key concern to free software advocates considering patents and other intellectual property that's owned by Microsoft. Richard Stallman has also referred to the use of Mono as "dangerous" due to such possible legal troubles that could ultimately kill this open-source project, which is owned by Novell, if such a situation ever cropped up. With these concerns coming up, the Ubuntu Technical Board has decided to reaffirm its position with Mono.
Mono is an integral part of Ubuntu with F-Spot, Tomboy, and other applications on the Ubuntu Linux desktop depending upon its C# / .NET implementation. At this time though, the Ubuntu Project has no problems with Mono and will continue having it be part of their main package set. With Ubuntu and other free software projects having not yet received any legal notices of infringements from Microsoft or other IP stakeholders, they see no problem with Mono continuing to be part of Ubuntu.
Scott's position statement on Mono in Ubuntu can be found on Ubuntu's mailing list