Putting iBank on Dropbox
Dropbox works for me using multiple computers with a single iBank file.  It's good to set iBank to automatically backup its files onto Dropbox also.  The only caveat is if you open your iBank on a second machine before the Dropbox has updated with the file from the previous machine.  In that case you'll have two competing iBank files trying to update on Dropbox. 

The fix is to disable backup/sync temporarily, delete the iBank.ibank file from Dropbox, and restore the iBank.ibank file from the backup.  Then enable the normal backup/sync procedure.

I rarely have to go through that procedure, though.  Just keep an eye on your Dropbox folder when using a new machine, and wait til it's updated before using iBank.