i have been doing alot of testing so far and this is what i have come up with.
i have set up a test domain to mimic my main domain with users ad and so on. on the test server i have installed ex2k7 and useing poweshell have imported the mail boxes (not easy to do but once you get your head around powershell and with alot of help from the net is not to bad), becouse the exchange server will not be forward facing i have also set up a isa 2k6 server as the edge then configured all the security certificates and forwarders to access the outside world (that bit was a pain as there is very conflicting info out there on how to do it).
the outcome so far is that we have both owa and full outlook access from anywere over a secure conection with only the isa server exposed to the net. the learning curve has been quiet high but myself and my colegues both have a good background knowledge on the systems.
the original question about the pro's and con's for slt is to get a general idea and thoughts and to make sure there wasnt anything i had missed.
If we do decide to go forward with it and it is cleared by slt my intention is to do a staged migration (staff first then year by year)
just to finish i would like to thank everyone involved in the discusion and to say your points will be put into the document to go forward to slt.
Loopy