When using Windows Live Messenger you will see a splash screen inviting you to use a 30 day trial. At the end of that period you will see another splash screen inviting you to pay to continue using the service.
The first markets this applies to are Sweden and the UK, but other countries will be included in due course. Some operators include Windows Live as part of their contract offerings (e.g. 3 in the UK). In these cases the charges are included in your contract and you should not be directly billed.
Charging only applies to the Messenger service. Use of Windows Live Hotmail or Windows Live Spaces will remain free. Charges will likely be made via premium SMS with a likely cost of around £2.
Charging for such a service may not be popular, but it is typical what happens for mobile messaging solutions. There are a number of free options which you can use as an alternative such as Fring, Palringo and Eqo.