Easy File Split and Join is a fast and free app to split or join files. This is especially useful for sending file attachments, recombining files downloaded in parts, and getting around file size limitations. It is opimized for writing very large files even on devices with very low memory.
UPDATE — Advanced multi-file join has been added! Arbitrarily join any number of different files.
Anecdotally, joining a 400 megabyte matroska video file took 50 seconds on my Viewsonic Gtablet.
You can download Easy File Split and Join from the Android Market here.
Update 4/2012 This is now open source and hosted on GitHub. Check it out at my GitHub page.