Some of my posts are "rants" and subject to Apple censorship (so I post more solutions on other non-Apple forums), but hopefully this one is not one of them.
I just grabbed my Family Pack iLife '09 and installed just iDVD on top what came with new Macbook Air (parts of iLife 11). There is no problem, but in order to make sure there is no clash I called numbers mentioned by Ziatron and spoke with sales and tech support about mixing versions (especially when you install older components over new code base of iLife). They said there should be none.
So far so good. There were issues with USB DVD drive connected directly to Macbook Air (???) but when the same drive (it is USB/Firewire enclosure) was connected to other computer with sharing over network the installation took about 5 minutes only.
Make sure that you just do not click on "Continue" in installer and pay attention on subsequent screens. One of them contains button "Customize" that allows you to select iDVD only (and some mandatory iLife support and who knows what, but supposedly it does not clash with newer code base, but you are not able to opt out/uncheck).
I still have spare licences for Family Pack iLife '09 so I think Apple does not mind this solution. What I would recommend to others is instead of spending money on iLife 11 (not much more over '09 except more bugs as i hear and nothing new in iDVD) then buy discounted '09 somewhere online.
iLife '09 works on Lion - that's what I heard on phone from Apple people.
Good luck.