I think that, after reading the very interesting suggestions here, that I'll go for the following setup. The game will start in a military camp, with the players part of a royal army; during the first few weeks they'll be sent out to do warlike stuff to the surrounding gnoll and goblin tribes. They gain a couple levels, look cool, impress everyone, and so on. Then one player, built for the purpose with the Torvsalan noble background, will be called into the royal council; they will be informed of their surprising inheritance ("Did you know that you are actually the fifth cousin twice removed of the last Earl? We did.") and given the opportunity to take over the castle. This is something of a mixed gift, since the castle has been derelict for five years, has become infested with Gnolls (like that sandstone castle in BG1) and all the tenants have fled. So they will have to remove the squatters, attract skilled hirelings to staff the castle, and make the lands secure enough to attract peasant farmers.
Naturally this will all take time, money, and good old questing. I like the suggestion by [MENTION=37579]Jester Canuck[/MENTION] for the hirelings to prompt quests of their own - the blacksmith asks that they help her re-assemble the famed Lost Breastplate of Plotness, that sort of thing. Basically they can serve in the same roleplay and quest giving role as companions in Bioware games. As the tenants have fled, there is no income from the castle lands, meaning that I can quietly ignore the accountancy side of things, at least at first. And, having played a fair bit of Pendragon, I can see all the benefits for downtime that this gives. The players will have a reason to allow the winter months to pass in downtime - they have a sweet castle to chill in. And then they get summoned to Spring Court, good place to meet and greet with the kind of people that give quests out...
One alternative option to inheriting the Earldom is, I guess, to go for the shared tenancy option suggested by [MENTION=996]Tony Vargas[/MENTION] which allows for the group dynamic to stay normal; the king offers the castle to them as heroes, to tend over and rule for as long as they live. I don't normally like 'adventuring' to be a recognized profession, but it wouldn't hurt in this case. That way it can also be given some gloriously goofy title, like Griffon Lodge or something. [MENTION=2518]Derren[/MENTION] is totally right to voice concern over random strongmen wandering in and stealing royal land, and I'm keen to avoid ruining the verisimilitude of a semi-feudal setting.*
I have asked my players if they are interested in this. I don't see any point in trying to spring a surprise like this on them, since it would be fairly fundamental to how the campaign plays, and they could very rightly complain that they didn't sign up for this style of campaign.
* Fun fact: I work in a History department, and the medievalists get very upset if you use this word. Apparently it is a modern word that describes so many different legal and social structures as to be essentially meaningless.
