Well, I said condoms but it can be other types of small containers... These guys have plenty of imagination, probably more than I do!
Condoms are used because they're easy to buy, easy to fill, comparatively easy to swallow, comparatively easy to get out, and probably more difficult to detect.
More solid containers (as the ones required for the unstable explosives that you don't need to mix, but which will go with a huge bang anyway) can probably be swallowed, but there's little guarantee that it won't get stuck in your intestines. That is, if it doesn't go off while swallowing.
But sure, people who want to blow a plane up, can probably manage to do so if they want to spend the time and effort to come up with a solution.
Bruce Schneier makes some very good points, both on what is and will be possible, and how we should react.
The only reason I wouldn't want to go on a plane right now, is that I would want to bring my photo gear, and there's no way I'm going to do that under the current rules.