It looks like there is a bug when using the prefix and suffix fields together.

Entered "ns" in the prefix field and saw a bunch of domains. Added "ing" to the suffix field and nothing showed up. I figured the site was just slow until I realized that it wasn't going to refresh.

Cleared "ing" and typed "g" in the suffix field. Showed results. Then added an "n" just before the "g" to get "ng". Less results than before as expected. Then added an "i" just before the "ng" to get "ing".

That gave me a list of domains starting with "ns" and ending in "ing".

(Using Firefox 3.5.7 if that matters)