Ubuntu EC2 EBS images don’t come with swap space configured (for 11.04 at least). The “regular” instance-type images do have a swap partition, though only 896 MB.
If some process blows up and you don’t have enough memory or a swap space, your server could come to a crawling halt for a good while before the OOM (out of memory) killer kicks in, whereas with swap, it merely gets slow. For that reason, I always like to have swap space around, even with enough RAM.