A virus has been discovered which infects .NET executables. The virus is harmless it would seem, and seems to be another proof of concept virus.
There is a 1 in 10 chance that the virus alerts a user when an infected program is executed.

Quote from ENT News
Thankfully, the potential impact of the W32/Donut .NET virus is mitigated, antivirus vendors say, by its relative harmlessness. W32/Donut infects other .NET executables (.EXE) but does not stay resident in memory. And unlike many of today’s viruses, it won’t attempt to exploit a user’s e-mail to distribute itself, but must be distributed manually or downloaded from a Web site or network share. Finally, antivirus vendors say, W32/Donut doesn’t do any other damage to a user’s local PC, either.
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Virus writers take an early crack at .Net