OS X V10.7 Lion :: Can't Wake It Over Wifi (or Ethernet) Using IPad And Home Sharing

I've upgraded to Lion and everything's fine apart from one problem: using my iPad to access home shared iTunes content on my mac no longer works when my mac is asleep. That's a change in behaviour from Snow Leopard; I used to be able to sleep my mac and then wake it over wifi using either my iPad or iPhone to access all of my home shared libraries.  The mac screen used to light up for ten seconds and then go dark again, but it served me the files. 

Lion seems to go into a deeper sleep which it can't be woken from.  If I attempt to wake my mac from my iPad within about ten minutes of putting my mac to sleep then home sharing seems to work and lets me use the home share libraries, but if I leave it any longer than that then the mac seems to fall into a coma and can't be woken.  The weird thing is that I still see a "shared library" folder on my iPad, but when I click on it nothing happens.  I think my Time Capsule is listening for wifi access and issuing the magic packet to the mac, but the mac just ain't listening.   

I'm using a Time Capsule in bridge mode from an ADSL modem and I've accepted a TC firmware update last week and I'm on lion 10.7.3.  I've tried using Ethernet to connect the mac to the TC (and temporarily disabled the wifi connection) but that still doesn't work. Also, note I'm NOT suffering from the problem that 10.7.3 seems to be giving a few other people - my mac reconnects to my wifi just fine when I wake it up manually. For the record, This was an upgrade, so there's still a tick in "wake on network" in power settings and home sharing on iTunes is still configured the same as it was before. 

Info:
iMac 27" i5-760, Mac OS X (10.6.7)