I dual boot ICS off the SD and WP7.5 off the NAND. To be honest I am 99% of the time booting the ICS Android because WP7.5 just doesn't have the apps I use all the time! I mean there isn't even a proper multi vendor IM app! There is something called GChat which is supposed to be like gtalk but it just keeps stopping so I gave up on that. So I just boot WP7.5 when I am bored and I want to see if they have actually got round to releasing any decent apps yet.
Anyway I installed the XBmod-yuki WP7 and then I used his autoupdater from this location http://xboxmod.sylvester20007.com/CABS/OSBUILD.CAB/7004
to 8107.autoinstall.rar and even though it takes ages to get to that point it does work just don't pull the USB out of the phone until it says it is finished bearing in mind it does reboot many times.
To get the dual-boot working off the SD I followed this link http://forum.xda-developers.com/show....php?t=1250869
because I couldn't get the SD to work using the manual partition method I just used the cab that does it all for you and it worked.
But like I said I am contemplating putting Android ICS on the NAND and just getting rid of WP7.5 because it just doesn't do what I want and until it does EVERYTHING