I'm currently researching WEP cracking and wireless security protocols (just wanted to say that since I hope it established some credability).
WEP is weak enough that it can be cracked easily, but there are a lot of different factors that have to be in place. First off, you need all the equipment to interoperate with each other. In short, this normally means that you have to be running a distro of Linux, use a crack program in there (Kismet is the most common), have compatible drivers, a wireless card running a specific chipset configuration, and possibly high-gain antennas to monitor possible signals. That doesn't take into account whether or not you are going to do this with 1 or 2 laptops, and also doesn't take into account some other things (such as distance from the access point, number of users, etc).
In order for someone to hack it quickly, they'd need to be next to it, there would have to be a user already connected that is transmitting large amounts of data, and you'd need to know specific information regarding the network (does it use MAC filtering, for example). Granted, Kismet can generally handle all of this by itself - it can get channels, user names, MAC addresses, cloaked SSIDs, etc.
So let's assume you have all of that in place.
You begin the attack, but you have to reach a certain number of IV ("Initialization vectors," which is a fancy way of saying a readable/sniffable packet of wireless information). For 64bit WEP it's roughly 100-200K, which will be roughly 160 megs of information. 128bit might require onwards of 1-2 million IVs. Getting those is going to take a while if you go through conventional means - i.e., you let your computer sniff the network and monitor traffic. I'd suspect you'd have to wait between 1-2 days before you could successfully decode the message.
Of course, this can be bypassed by getting "replay" programs that essentially bombard the AP will packets, which essentially causes IVs to be generated much more quickly, expediting the process. I found a movie of someone doing this, and they were able to capture the network within 10 minutes. Really interesting.
Anyway, the problem with all of this is that WEP is vulnerable, BUT this is offset by the fact that you really need to be an above-average computer user in order to understand and run all of the necessary software. I consdier myself pretty knowledgable about them, and even I know that right now, I couldn't explain this stuff to most people, it would be way over their heads.
The workaround to this? Get the Wifi adapter. Then you just bridge into the DS without compromising your WPA.
Still, it takes some time (at least 10 minutes, at the least), a bevy of equipment, and a lot of technical know-how in order to crack WEP.
You can always turn off your SSID broadcast, but Kismet can detect it nonetheless. Still, doing so will stop some people who are just searching around blindly. Turn off the beacons and my guess is that only the most technically saavy users will find your network then.
You can do MAC filtering, but that isn't very strong either. I personally know of a program I ran across that lets you dynamically change your MAC address through software, meaning that if a network used only that as its security, I could break through it in about 3 minutes, tops.
So recap:
1) WEP cracking is easy to do if you know how to do, have the equipment, are in range, and can interpret programs. But this is a huge hassle by itself.
2) Get the wifi adapter. I'm really hoping NIntendo will release it bundled with Mario Kart.
3) Use the highest WEP bit encryption if you can (128, 26 hexadecimal characters), turn off your SSID broadcast, and enable MAC filtering.
I don't know anything about WPA, I might research that tomorrow.
I think as technology progresses, people will naturally move towards higher protocols - WPA, for example, with LEAP authentications, etc - and leave WEP behind. And at that point, peopel can just use two APs - one running a 802.11b network for small access devices such as the DS, and higher ones running greater bandwidths and security protocols.
So I don't think of it as a problem. I'm getting the adapter purely to support Ninendo and send the message that they are doing the right thing. So I'll just bypass security issues and use it as opposed to another card or reconfiguring clients. That's not a totally acceptable answer, but it is the easier.