Do you think it being a WD Green Drive would be a problem? Too slow for a shared iTunes library?
Not at all, all the "Green" drives I've had, the WDs and Samsung EcoGreens, have been perfect for home theater use and media sharing. I have no experience with the Drobo, never trusted it, but my slowest connection--a green drive as a USB AirDisk drive plugged into my Airport Extreme--is fast enough for a blu ray rip.
If I were you, I'd consolidate your media into 1 large iTunes home sharing library, then keep it directly attached to your Mac in the Lacie FW800 enclosure, a second copy of it on the USB drive, which you connect only to do periodic updates, then a third copy of it on your Drobo. (Since everything might not fit on a 1TB drive, just open up that LaCie and put something bigger in.)
On a side note; I don't have an ATV so I have not kept up but I thought when it first came out they were advertising Hulu+ but not Hulu. Did Apple reverse on that?
Conventional wisdom posits that Hulu is a threat to iTunes media purchases/rentals on the Apple TV, more so than Netflix.
You can do Hulu+ on the iPad with an app or in a browser on a Mac, you've never been able to do any Hulu on a stock Apple TV because it doesn't have user-configurable "apps" or a browser, though once people figured out how to get XBMC running on an Apple TV, plenty of folks have been using both the free and paid versions of Hulu on their Apple TVs.
XBMC running on the first or second generation Apple TV is quite nice.
But, there's also no reason why you couldn't keep the Mini right at your main HDTV in order to do Hulu+ duties until you've completely worked things out. It can do server and playback duties just fine.