The UKBA has issued a statement of changes to the Rules relating to visitors. They are trying to "combat" what they call "de facto residence" (ie. living in the UK on visit visas).
As of 6th April, there will be a new requirement that the visit visa applicant "does not intend to live for extended periods in the United Kingdom through frequent or successive visits".
For most US Citizens who don't require a visit visa in advance, this isn't a direct requirement but it may be that entry is refused at port if successive visits are made to the UK, experiences of which we have heard of on UKY.
Another amendment is that, if visiting family or friends, they need to be present in the UK at the time of the visit and if that family member/friend is providing finances/accommodation, they need to demonstrate that they are able and intend to do so. Where this comes into play is if you're visiting, say, your brother in the UK and another person, say, your aunt is providing accommodation, then your aunt needs to be in the UK at the time of the visit as well.