I've Googled and searched the market and can't believe there's no such app. Maybe some other SMS management app has this simple function, to unsplit messages longer than the 160-character limit that were broken into more than one part? Handcent and GO SMS Pro don't mention it.
For some reason (see the answers posted before Feb 17, 2013) people don't quite seem to "get" the problem. Here's a screenshot:
For whatever reason (incompatibilities between carriers in following the Concatenated SMS protocol etc.), I receive messages split in parts of <160 characters. What I'm looking for is a very simple app that would let me select two messages and join them into one, specifying which should be first.
SMSes are stored in a SQLite database, and it would be pretty trivial to append to one field the the contents of a few other fields based on a drag-and-drop interface. I'd code it, but I'm not experienced with Android development, and others could make such an app far faster.
I posted a request on xda-developers but the bozo admin there moved it and nothing happened.