It can be done but personally its not something I like doing.
Usually when a thread starts turning into a different topic its something coherently related to the first topic and you can see where the discussion spinned off to another. I think its better to leave the topic alone and let it evolve into another good discussion rather than cut it off. Moving posts around gets confusing and annoying(for the poster and the mover

).
On lots of forums they have topics where they just combine all they're random topics together and instead of being an interesting thing to read as a whole it just makes no sense. That to me ruins the feel of the community/forum overall. Being worried where something is posted is sorta silly in my mind, but I know some take it really seriously.
An example of a thread I wouldn't split would be if someone asked about wireless routers and it evolved into a discussion about hardware firewalls and which are better/worse ect. Now if the topic was about routers and a disscusion about music broke out in the middle then yeah that thread could be split. To me its just better to let discusions continue then try to control them. After all at this point I think most of the regulars here just come here to talk to one another anyway, with a little tech help on the side.

To be honest when I see someone ask for a thread to be split in the middle of a topic I usually forget because I will respond to the thread and then wait for a response to my post and in the rush I just forget. If you honestly feel that a thread just must be split send me or TwistedMetal a PM saying where to split the thread at and what to call the new one.