There are three variations on this phrase that I can find in Google, and I'm not sure whether all three are really used, or how frequently. But I am confused about the differences between them.
大{たい}したことではない
I believe this means "(It's) not a big deal." Ok, I understand.
大{たい}したことはない
Isn't this different from the first sentence? It seems to mean "There aren't any particularly big problems."
大{たい}したことない
This one could go either way.