Unless ZoneEdit has changed it's policies recently, they stopped doing immediate updates to their zone files a couple years ago and now only update them about 4 times a day.

We used to host about 100 domains with zoneedit, but the combination of their taking away one of the best benefits they offered (near-immediate zone file updates) and some major failures that caused both primary and secondary nameservers to go down at the same time caused us to move to hosting our own DNS.

Don't get me wrong, Erik and company at Zoneedit are nice people and provide a decent service, but I think there are better options now. For one, we've found that Enom's DNS (free for domains registered with them) is rock solid reliable, comes with something like 4 redundant nameservers at no additional charge, and updates are, in fact, immediate (barring the TTL delay that Luke mentioned.)