At least that's what says this story run by Engadget Mobile:

App certificates have long been a bane to S60 users and developers alike, causing pain, frustration, and an almost obligatory cash outlay to get your hard work certified to run on the very platform Nokia is so quick to call "open." Finally, it truly is, thanks to the hard work of the Symbian hacking community that has developed an easy (or easy sounding, anyway) method of "jailbreaking" the Symbian 9.2 device in your life (S60 3rd Edition FP1 users, that's you). After that, installed apps won't need a certificate at all -- let alone an invalid one -- to do their dirty work. Open, indeed.

It points you to this site where instructios are given to unlock a single phone using AppTRK and a couple of Python scripts. The hack isn't permanent, which means after a reboot your device gets back to normal. They say they're working on a permanent hack, which would allow iPhone-style jailbroken firmwares running unsigned applications at full capabilities.

I haven't tested it myself, but it seems it works at least on Feature Pack 1 devices. I will test with my N95 with an application which requires all capabilities (including manufacturer-given) to see what happens.

Anyway, I would advise nobody to do it to their own devices, since this opens up the possibility of you installing apps whose authors you don't know and can do real damage to your phone if they are making use of sensitive capabilities whose use is protected by the whole signing scheme.