If its just a date no its not cheating, but just yourself in your girlfriends shoes if she was to have a date with some bloke while you're on this 'break' and also is it really fair to get this other girls hopes up until you know what the situation is with you and you girlfriend I dont think its really fair to involve other people. Beleive me I did the same thing while I was going through a bad patch and I hurt the other bloke alot.
If you really think there is still a chance with your girlfriend I wouldn't put myself in the situation where the date might be seen as cheating.
Not much help I know, but as 4getmenot has quite rightly said there are 2 girls involved here & you don't want yourself seen as a bad guy 2 times over!
Best thing is to think about how you would feel if your girlfriend did the same...:o)