I used a Panasonic FZ8, a camera known as an "ultrazoom" because it provides an optical zoom lens with an equivalent field of view of 35mm (moderately wide angle) to 420mm (nice telephoto). One feature of the Panasonic ultrazooms is something called a "telemacro". What this does is provide a partial macro feature at the full zoom of 420mm. The result is that a 4" wide object (for example, a butterfly with a bit of background) can fill the viewfinder, all at a convenient focusing distance of about 33".
Below are a several of my favorites from that outing. Each was taken without a flash, and the camera was hand-held. The problem with the Panasonic is that because of its small sensor, the noise level at even ISO 200 is a bit on the unacceptable side. I used software to reduce the noise. Also, due to the smaller sensor the depth of field is greater than a dSLR, which can be good but also means backgrounds are more in focus and therefore often distracting.
The second butterfly house is in Deerfield, MA and is called Magic Wings. This winter I went there during normal hours and had a difficult time because the butterflies seemed to be in constant flight. Below are a few of the images I was able to get.
This time I had an Olympus E-520 dSLR with a 70-300mm zoom lens. This lens provides an excellent macro at full zoom. A 1.5" subject will fill the viewfinder. And at 150mm, where this lens is known to be sharper, a 3" subject will fill he viewfinder. All this at a focusing distance of about 40".
My camera was set at Shutter-priority, with Auto ISO. I used 1/500th second shutter setting but will want to try 1/250th next time so as to use a lower ISO. My exposure meter is set to center-weighted. I use EV compensation depending on the darkness/lightness of the subject butterfly. Again, no flash was used, and the camera was hand-held.
Note that the backgrounds on this second set of images are blurrier and softer. This is because the depth of field is so much less on a dSLR at equal aperature openings.
Finally, I'd like to mention a good online resource, http://www.learnaboutbutterflies.com/, which also has a section on butterfly photography.