Well, most of the time, when you hear about a CFL bulb flickering, it is happening when the light is turned on, and just not getting enough power to operate - such as when one is placed in a light fixture with a dimmer switch. If you want one that can be dimmed you have to purchase one specifically for that, a 'dimmable CFL'.
CFL's have a gas mixture inside the glass coil. An electrical charge is ran through it, to 'excite' the gas, causing it to glow, or light up. There is a specific process to this, and this is why they have those large bases - there is a starter built into the bulb. It takes a bit more energy to 'start' the bulb, but once it is started, it runs at a lower energy consumption. Even the 'starting' of the CFL bulb, though, will use less energy than a typical incandescent bulb. Ok, back to the gas inside the glass coil - while there may not be enough electricity to 'start' the bulb, there may still be enough to cause it to flicker.
Other reasons could be cold temperatures, or maybe even the occasional bad bulb. (By the way, this gas is 'inert', and safe, not flammable like other gases, such as natural gas, or propane. It is simply a mixture of particular gases, like argon and neon, that are more sensitive to the electrical charge than others we breathe all the time)
Every recommendation I've seen, regarding a flickering CFL, says to first try a second CFL in the same fixture, to see if it also flickers. If it does, then the fixture needs to be checked (assuming you are not trying to run a CFL on a fixture with a dimmer switch). I would go a step farther, and try the bulb in a different fixture. Either method... trying a second bulb, or moving that one to another fixture, will tell you if the problem is the bulb, or the fixture. I would try this before trying to return the bulb for a refund.
I currently have at least 8 fixtures with CFL's, none with dimmer switches, and they all work fine. I do notice that cold temps will make them appear to be 'dim' upon first starting. I've never had one that flickered, ....yet, hehehe.
While it is doubtful the bulb itself is a fire hazard, you are right to pull it out, when questioning its safety. Try the bulb in another fixture - a lamp for example. If it continues to flicker, send it back for a refund - better safe than sorry. There is the possiblility, that there may be a small current flowing through the fixture even though the switch is 'off' - this would indicate a wiring problem, which should be addressed as soon as possible. This is unlikely, if you've been using the fixture for some time, with no other problems - but it is something that may need to be checked, if multiple bulbs flicker.
Occasionally, you will see a bad bulb - and it would just be best to simply not use that particular bulb. You need to rule out that there is a problem with the fixture though (again, by trying other bulbs in it)
If you see other bulbs that flicker in the fixture, then I'd recommend calling in an electrician to check the circuit. (Again, need to caution, the common CFL is NOT intended to be run in fixture with a dimmer switch)