There is a new worm circulating round the Microsoft IM world called Kelvir-ASVID which is a variant of the Kelvir worm. On September 9th it started to work its way round the Reuters instant messaing platform.
This is seems to be an every increasing issue in the world of instant messaging as virus writer move from using email as a mode of transport to using IM platforms as the means to distribute themselves. It is fortunate however that currently the large IM services e.g. AOL, MSN and Yahoo have little or no cross platform support so this has kept most virus' contained although I personally think that as we move towards using SIP across all the services and then start to provide connectors between the services that this could become more of a problem.
For those of you that are looking to deploy Live Communicator Server I would not let this outbreak affect your decision to purchase or not. Virus' will always be a problem and with the right design, deployment and relevent secuirty measures it is possible to make your IM infrastructure secure. I recommend taking a look at the security paper on http://www.microsoft.com/livecomm for more information.