I recently received a question asking how to move music back and forth from Sibelius to Finale.  I too have this problem quite often as almost all of my old lessons and beginner music that I wrote out for my classes was done in Sibelius but now I would also like to be able to use it with SmartMusic (which requires Finale to create custom arrangements).  Since neither company wants to allow the other to begin reading and writing their own proprietary formats, the grand dream of one day having an open, comprehensive transportable music file format is probably never going to happen.  For now there is a format called MusicXML but it leaves a lot to be desired when working with complex scores. Moving a document from one platform to the other using MusicXML will get the notes moved, but other annotations often are misplaced or left out altogether. 

Open Source Solutions To Music Notation File Formats
Some of the more forward thinking solutions to this problem that I have personally seen are open source formats like Lilypond and OpenScore.  Lilypond does not seem to be gaining much traction, but OpenScore is actually supported by some fairly big names like MakeMusic (Finale's parent company) so at some point hopefully these new file formats will be incorporated into future versions of the software.

Converting Files Between Finale and Sibelius
For the time being the only way to go back and forth between Finale and Sibelius is to use the older MusicXML file format.  In Finale 2010 you can save and import MusicXML files.  In Sibelius you can import music from the MusicXML format but for some reason they have yet to incorporate an export feature.  To export MusicXML out of Sibelius you are left with only one option, to use a program called Dolet to convert the Sibelius file over to MusicXML format.  Of course that conversion comes at a cost of $199 for the Dolet software.  If you have only one or two scores to move from Sibelius to Finale you can also just ask Recordare to do the conversion for you, at a cost of around $20 or so per score.

If you can sense the sarcasm in my writing here you are not mistaken.  Recordare has every right to charge whatever they wish for a service that obviously fills a very important need.  My issue, sadly, is with Sibelius for not just incorporating the MusicXML export into their products by default.  MusicXML as a file format is now five years old.  Come on Sibelius, you know people want this feature and yet you continue to leave it out of new versions of your software.  If anyone from Sibelius happens to be reading this I would love to get a comment or two about this oversight and if there are any plans to eventually remedy it.

So there you have it.  The only way to convert from Finale to Sibelius is to use MusicXML export from Finale and then import it into Sibelius.  The only way to convert from Sibelius to Finale is to pay for Dolet, use it to convert Sibelius files into MusicXML and then import the MusicXML into Finale.  In almost any case you are going to run into some snags along the way, so it is probably a better idea to simply start writing in the target format instead of having to convert it later.