I have seen the same thing with my signal when using the browser. Even with 0 bars, data still downloads, so I've never really been sure if it's the actual signal reception that fluctuates or if it's just the meter.
Freebie: You can change the bar display to a number (and back to bar display) by typing Alt+nmll on the home screen. This reveals quite a bit more information: "no- signal" occurs at -256 dBM, and you will see that the 0-bar level never drops that low, so there must be some signal. Lowest I've seen mine is -110 dBM. Here is an approximate conversion chart:
1 bar ~ -102 to -120 dBM
2 Bars ~ -93 to -101 dBM
3 Bars ~ -87 to -92 dBM
4 Bars ~ -78 to -86 dBM
5 Bars ~ -40 to -77 dBM
Posted from my CrackBerry at wapforums.crackberry.com
It's a combination of the handset not always generating enough power to accurately receive the signal strength data, and more often a host of frequency interference items, from sunspots to microwaves. Any device will effect it, given it's proximity and transmission strength. Just as your device effects your television, or radio if you use it at a close enough range.
I did realize that data still downloads when it shows 0 bars, i was just hoping I didnt get a dud and would have to bring my phone back, since i've only had it for a week.