Just reviving this very old thread! It seems Ryanair have actually now (finally) lanched on around 20 aircraft (mainly Dublin based ones!)
Ryanair.com - News : Launch of Europe’s 1st Fleet Wide Inflight Mobile Phone Service
Has anyone been on one yet? Any reports as to how it works/network it uses. Surely there will be cheaper ways to use it than the prices mentioned in that article!
Incidentally..
http://www.gsmworld.com/roaming/gsminfo/roa_xaoa.shtml reports that there aren't that many roaming agreements yet. e.g. For UK, only o2 customers can use the service so far (also mentioned on the Ryanair website). German users only o2 and T-Mobile. For us here in Spain: absolutely none so far... so I will stick with my o2 UK!

Not that I think I will ever see reason to use it other than a few SMS's.