Solaris 2.6 reaches the end of Vintage Support Phase 2 (EOSL) on 23 July 2006 (see InfoDoc 10012). This means that from 23 July 2006, Sun will not offer you any support on Solaris 2.6 beyond diagnosing hardware problems, unless you throw money at them and purchase a special vintage support contract for Solaris 2.6. This sounds harsh, but remember, Solaris 2.6 has been EOL since 23 July 2001 and Sun is trying to make money. Many other companies don't even offer 5 years support after EOL.
If you're worried about your applications being compatible, fear not. Provided you only used the public interfaces (those documented in man pages and on docs.sun.com) within your code, there is no good reason why your applications won't work on Solaris 10. Obviously, recompiling on Solaris 10 will give you the best performance.
Solaris is also the ONLY operating system out there that guarentees binary compatibility with all it's releases (backward compatibility only obviously).
So, if you're going to take the leap - check out the upgrade tips Sun offers and be awed by the shear brilliance of Solaris 10.
If you've got a spare machine, then definitely check out OpenSolaris and tinker with the stuff coming to Solaris is the future.