Migrated to new Lion, how to share Time-Machine?
No. Most technically minded people have stopped trying to hack together AFP as Apple have changed the code to tighten up and only show legitimate sources not connected as direct attach storage.

Apple provides two products that let Mac Clients see a hard drive as a Time Machine backup when they are not connected DAS but instead NAS (network attached storage) which facilitates the sharing of one drive with multiple macs simultaneously and without reconnecting cables:


TimeCapsule - only the internal drive is supported for Time Machine.
OS X Server - can mark any share point as being available for Time Machine use.


Yes, anything is possible in software given enough money, time or ingenuity, but for reasons I can only presume is enough smart people breaking TimeMachine and blaming Apple for lost data when using DIY Time Machine targets, this has been tightened up in Lion to preclude any easy solution that is documented at present on the internet from working.