If it's not https, don't provide login credentials to that site. It's the same on the iPad as it is on pretty much everything else.
While that is certainly true on a secure local network (hopefully your business, your home (you ARE using WPA2 on your wifi network at home, right??? WEP or WPA is NOT enough), etc.) additional steps are needed on a public network. You go to some coffee shop, college campus, hotel, airport, etc. and join a public network and visit all the https you like, every username and password you use COULD be stolen if there was a simple man in the middle or other types of fairly simple attacks sitting on the network.
Unless you take extra precautions, on an open wifi network (coffee shop, campus, etc.) I would not do anything other than surf non-password websites. Definitely not banking or buying anything at Amazon or wherever. You can judge yourself for email, some say email accounts are not worth the effort. I agree it is less of a valued target, but a) I still wouldn't want some hobby hacker poking around or using it as a spambot, and b) are you sure no other account information (from amazon, your bank, etc.) isn't traceable in all your old email? I hope not, but still....
So what can you do if you are on a wifi network and want to check your email or whatever? I have and would suggest you turn on a VPN (stand for Virtual Private Network) and the iPad supports many (Cisco's, PPTP, and I think with OS4 SSL) VPNs. You can set up your own if you want to have your computer or a server do it at home, even free stuff like hamachi can do that. Or you can use some of the questionable free third party stuff out there, but then you are trusting that third party. Two relatively cheap for pay third parties (which have been around for awhile, so should be trustworthy) are hotspotvpn ($100ish a year?) and witopia ($40ish a year?). Takes 15 minutes to sign up with one of them, enter in the info on your iPad (iphone, laptop, whatever), and have it when needed. When needed it is as simple as popping into prefs on the ipad, sliding the VPN slider to ON and let it connect over the next 20 seconds, and you have a secure tunnel that somebody sitting on the hotel's network shouldn't be able to break. If they can, they have bigger fish to go after than you.
I would be interested to know of any other, TRUSTED, VPN solutions out there that people use???? Your employer may have one too, but then I don't know their policies on using their network for private use.
BTW, 3G isn't even as secure as it really should be, though practically it is unless you have somebody after you who has a few thousand dollars worth of equipment. The hobby hacker is most likely targeting the public wifi networks around you with free, easy to use software...not packing equipment that costs some money and requires expertise to use just in case they can crack 3G. I personally consider 3G safe, but it is not as safe as SSL / VPN connections over WiFi for instance. But for practical purposes, it is for now.
