UPDATE: We have decided to give up on iCloud. We simply can't make it work reliably enough. We are currently in the final stages of developing our own syncing solution which will be available soon. Please stay tuned.
In the meantime if you really what to dig into this:
If Instacast iCloud Sync does not work, you can check a number of things that might prevent it:
Also Apple has announced that there are bugs in iOS 5 related to syncing documents over iCloud which will be fixed in iOS 5.0.1.
UPDATE: It seems there are still bugs in iOS and Mac OS X that prevent the iCloud daemon from syncing data. If it does not work for you, please go ahead and kill Instacast on every device. Then delete the Instacast.icast file everywhere and thus resetting the sync (also on your Mac!). Then start Instacast on one device and wait 30 seconds. Then check if a new Instacast.icast file has been created. Then check on every other device if the file has been synced over by iCloud. Then start Instacast on the other devices.
UPDATE: If you're deleting the Instacast.icast file and the system is telling you that some changes are pending, this means that the iCloud service is stuck somehow. I couldn't yet figure out why, but I have reports that deleting everything and starting over solves the problem sometimes.
UPDATE: Make sure you don't have an old mobile.me account attached to your "Mail, Contacts, Calendar" settings. These settings seem to interfere with iCloud as well.
UPDATE Dec 14 2011: There is a new hidden diagnostics panel for iCloud Sync in Instacast 1.4.4. Tap on this link on your iPhone or type 'instacast://diagnostics/icloud' in Safari and Instacast will pop up a view that displays 4 checkpoints that need to go green in order for the iCloud Sync to work. If it doesn't do that, some settings are false or some errors happen on your device. Tap on 'Send Diagnostics Report' and email it to support[at)vemedio(dot]com.
UPDATE Dec 16 2011: There are typically 3 types of problems in order of appearance:
UPDATE Dec 19 2011: Concerning Issue #3 above, I found the root cause of a problem that sync is not even trying to start. It has something to do with a false evaluation of internet availability. This issue will be fixed with the next update.
Tell us your iCloud Fail Story and send us a bug report.