I am running the latest version of ical and lion on a new £2K MacBook - I have a hosted MS Exchange 2007 account - and this issue is driving me nuts. I have spoken to Apple Care (less) who have advised me that this is not really an issue as they have seen very little traffic on it. Apple Care went on to tell me they did not support exchange and that their software was only compatible with Exchange 2010 which rules me out! I notice that some of the posts indicate that they are using 2010 and still experiencing the issue which makes a mockery of their comment. I found them complacent and not that interested. I have used Mac for two years with relatively few issues - but LION is a disaster with issues experienced with printers, iPhoto Themes, iPhoto importing of pictures and on and on.
I have contacted my Hosted Exchange provide (Cobweb in the UK) who reported that they had received no calls on this and that they would not help making any transition to 2010 - not that it appears that would make any difference.
Buying a new MacBook and upgrading my other to LION was a huge mistake ..... if anyone comes across any solution please advise. In the meantime I will have to put the appointment details in the location field and then edit and resave when the event has reverted to New Event ... once you have resaved it, it appears to stick (thank god).
Sort it out Apple and stop launching unstable systems - the whole reason people move to Apple is because they are meant to 'just work' - 2 years ago I think this was true but now I am not so sure. Lose this USP at your peril.