You may have already thought of upgrading to the latest iPhone OS version, 3.1.2.
I myself have completely skipped version 3.1 released in September because both of the bug reports (see for example THIS) and the problems I faced during trying to upgrade on both my iPhone 3G and iPhone 3G S.
Now that the (otherwise, not very good) third-party browser BOLT (not to be mistaken for Bitstream’s excellent, Java-based browser on other, Java-enabled mobile platforms!), with the latest version, stopped supporting operating systems prior to 3.1 and the bugfix version, 3.1.2, was released (along with a reliable way of jailbroking it), I found it, at last, necessary to upgrade.
As far as the OS 3.1 iPhone 3G Voice Memos problem is concerned, it’s still not fixed for the 3G (as is both reported HERE and tested by me). While the recordings in the 3G S are unlimited (I’ve tested with a 2:10 recording), not so with the 3G. I don’t know whether it’s an oversight, or Apple introduced this bug to force present 3G users to switch to the 3G S. (Many say OS 3 runs definitely slower on the previous-generation iPhones and iPod touches so that Apple can force more people to switch.) I don’t know whether this is true.
The new, 3.1 version (strictly for 3G S users) has also introduced voice control support for Bluetooth headsets (both traditional mono and stereo ones). This, unfortunately, has also some consequences: one of the stereo headphones (the Plantronics Pulsar 590) I’ve tested has become completely useless under the new operating system version as is also explained HERE. Unfortunately, the Gear4 BluPhones have lost their ability to initiate calls on both models. The same has happened to most? all? the Motorola stereo headphones I know – the HT820 and the S9 / S9-HD on the 3G S (but not on the 3G).
If, for some reason, you don’t want to lose the ability to use these headphones (or, on the 3G, the unlimited recording with the built-in Voice Notes), you may want to stay away from upgrading. If you’ve already upgraded and would like to downgrade (to 3.0.1), keep in mind that it can only be done on the 3G (but not the 3G S).
There is also a jailbreaking tutorial at http://forum.brighthand.com/showthread.php?p=1782991 , http://www.howardforums.com/showthread.php?p=13119665 , http://iphone.modaco.com/content/general-discussion/294427/iphone-os-3-1-2-report-jailbreaking-tips/ , http://www.mobilitysite.com/boards/apple-iphone-forums/267016-iphone-os-3-1-2-report-jailbreaking-tips.html and http://forums.toucharcade.com/showthread.php?p=568625 .