First, let me tell you that I'm only a newbee so my message in here may be stupid. But I give it a try
As I have read/heard there can be problems when "upgrading" v2.5 to 3.0 - for example the template you are using may crash because it might not be ready for the 3.0. So I thought what about making a v2.5 AND 3.0 safe template that we know will work in both versions? Then when upgrading we can first install the 2.5/3.0 template and then do the upgrade. This way the site will not fall down; well atleast it should not do crash
Atleast it should be possible to access the /administration/ area after the upgrade. - Well, just an little idea I have been thinking of 
