The processor is actually a OMAP3430, which is spec'ed to run at up to 600Mhz. The default shipped top frequency was 600Mhz, not 550. One thing Nokia did do was over-volt the chip at 600Mhz by just a hair vs TI's suggested voltage for that speed. It's very probable they did that because the prototypes needed the extra voltage to sustain the speed, and it never got backed out.

Realistically, Nokia for the most part stayed in spec. Because of how the chips were made, some can in fact be overclocked up to as high as 1.2Ghz (a 2x of what they're rated at). Most become unstable running over 900Mhz.

Keep in mind, all of this is generally for short-bursts. If you look at standard usage, the device spends most of it's time in it's sleep state and maybe 5 to 7% of it's time at it's highest speed. The built-in scaling system is actually very good about keeping the CPU running at the lowest speed/power level needed to do what it's doing.

If you're using a CPU intensive program (doing ray tracing, running a gaming emulator, etc), then you may want to consider NOT overclocking while doing that, since that's they type of activity that will cause excessive use, excessive heat, and possibly cause damage. For short-term bursts (GUI updating, 3 or 4 seconds of intense computing for a flash app, etc), overclocking isn't an issue.