Thanks Apple for not making this backwards compatible - but I guess just like with your new "connectors" and power plug, its a way of forcing new equipment buys.
Usually iPhoto will give a warning, when it is opening an iPhoto library in an older format, that this will make the library incompatible with the older software. You should have stopped at that point. Apple's software is usually able to open older document formats, but there is no way that older software can work with new library formats, that had not been invented, when the older software had been written.
As to your network drive: You really have been lucky that it worked so long: Have you read the document I linked to?
It's recommended that you store your iPhoto library on a locally mounted hard drive. Storing your iPhoto library on a network share can lead to poor performance, data corruption, or data loss. If you use both iPhoto and Aperture with the same library, using a Mac OS X Extended formatted volume is recommended. For more information, see Aperture: Use locally mounted Mac OS X Extended volumes for your Aperture library.
Problem is that the new machine came with the new software, so reverting - i.e. installing the old operating system - is not an option
That is really a problem, if you cannot downgrade the new machine and upgrade the older machines. Keeping two separate libraries and syncing them is probably the best option, if you want to use iPhoto on all machines.
