Some advice I gave on getting to grips with creating Skill Challenges
They are more an art than a science. I always start with knowing what chance of success will produce a nailbiting win for the PCs:

# of successes needed / (# of successes needed + 2)

So for each Complexity, its:

1 = 66%
2 = 75%
3 = 80%
4 = 83%
5 = 86%

Translated to rolls on a d20 thats:

1 = 7+
2 = 6+
3 = 5+
4 = 4+
5 = 4+

Next get your table of all PC skills. You should be able to get a good feel for the gap between the Primary Skills and the likely PC skills to be used. If the gap is 1 to 3, the PCs will likely waltz it. If its over 8 then it is too hard. Generally, its best to err on the side of PC success than not, as it can be very frustrating if the bar is set to high.

Oh and remember to factor in Aid Another and any skill bonuses from Powers, Feats and Magic Items.

The real skill comes in creating a SC that doesn't just see the PCs roll their highest skills. Max successes, adding other bonuses to non-highest skills, and a few negative results in using inappropriate skills can help encourage players to branch out.