The Grumpy Celt said:
How much social mobility is allowed in your game?
A lot. My assumption is that in a garden-variety D&D campaign (that is, a "points-of-light"

scenario which features small kingdoms or borderlands beset by evils that provide convenient villains for the PCs), successful PCs who build themselves a good reputation will eventually rise to the ranks of power, unless they consciously refuse such status. In my various games, I've had PCs (typically rangers, rogues, peasant warriors, or secretive wizards) who embraced anonymity and adventured under various aliases/disguises so as not to attract attention, and thus never rose up in the social order. I've also had PCs become barons, kings, and emperors. Becoming a ruler is an excellent exit strategy, something that was perhaps more encouraged by 1e/2e rules (where XP required to rise in level became prohibitive and resurrection was never really easy past a certain point, so the reward/risk ratio diminished with level) than B/E/C/M/I (where domain ruling was built right into the system and thus didn't need to be a basis for retirement, and level advancement and resurrection were easier than 1e/2e) or 3e (in which you can advance rapidly enough at high levels that there's no real incentive to retire).
To put it another way, if a character is a peasant, does the DM ensure they will always be a peasant, regardless of experience points/level? (i.e. the character dies or is otherwise made unplayable, or the DM forces a new character to be created rather than allowing social status to be increased.)
The short answer is: It depends. Heroes are never peasants in my millieu; the moment that humble farmboy slays the orcs raiding his village, rescues the mayor's daughter from their lair, and liberates a fair chunk of wealth, he's at least a local hero, and probably wealthy enough to be a secure landholder and thus well-off by village standards. Further adventures (assuming he survives) bring more wealth and greater renown, and thus higher status. The PC could become an outlaw, criminal, or enemy of the state, but that's status along a completely different track from peasant to emperor.
In short, the way my (Forgotten Realms) campaign has traditionally worked, a PC is either going to stay entirely outside the system or achieve high status within it as he rises in level. A 15th-level PC could be a secret Lord of Waterdeep, a baron building a new realm in the Border Kingdoms, the master of a thieves' guild, the head of a shadowy (or open) wizards' brotherhood, or a patriarch commanding thousands of the faithful. That's pretty vanilla, granted, but I take my D&D old school.