RealtorRose, I apparently missed all the descriptions of how much dairy we could have when I started SBD, so I ate unlimited dairy (though I was more careful with lowfat cheese) items like fat free yogurt, fat free cottage cheese, skim milk, etc. and I still lost.
Check the ingredients on your cream cheese. If the fat free cream cheese is free from any scary non-SBD-safe ingredients, I don't see why you couldn't have more of it.
Remember, nonetheless, that even though we don't officially count calories as part of SBD, the fact remains that how many calories we eat is really the key to losing weight (along with how many calories we burn, aka exercise). SBD works for some because it removes the cravings that lead to eating too many calories. If we overeat on the things that are allowed on plan (for emotional eating reasons or otherwise), we'll stall our weightloss, even if we don't have cravings. So watch how your weight does with the extra cream cheese. If you don't have any problems with it, go for it!
BTW, another tasty alternative with celery is Laughing Cow Light cheese. It even comes in flavors, and it comes in these little pre-wrapped portions so you aren't tempted to eat too much.