We know saying goodbye can be hard, but don't worry. Not only will all your Messenger contacts appear in Skype when you upgrade and sign-in, but you can also redeem a free welcome gift. Follow the instructions below to enjoy calls to landlines and mobiles around the world, group video calling, group screen sharing (and more) absolutely free for a month. We want to thank you for being a Messenger customer and even more, are excited to welcome you to Skype!
So for those of you that don't know, Skype isn't just PC->PC, it's full-on VoIP with landline and cell access too, if you buy Skype credit. Apparently it's pretty cheap to call phones from the PC program (2.3 cents a minute pay-as-you-go anywhere, or $3 a month to "unlimited US and Canada"), and with a different subscription you can even get a number with skype that people can call from their phones.
This free offer though, appears to just be PC->Phone calls, to further convince users to switch. You sign into your Microsoft account with your MSN details, then visit the link they sent you in your e-mail if you got one (which, humorously, has "skypewithlove" in the URL) and somehow redeem the free month of credit (when signing up for a recurring payment like normal). I assume this is only for your merged Skype+MSN account, I don't know if they let you redeem it on an existing Skype account.