As you all know, iPhone OS 4.0 beta 1 is released to developers. I will not cover all the new features of iPhone OS 4. Those posts are all over the internet.
But I will give you the standard warnings:
1) This is beta. If you are not a developer using it to develop apps, do not install it.
2) Like all previous major betas, this will probably expire every few weeks requiring you to restore or update.
3) This beta does put a new baseband on your device. If you ever want to unlock, dont touch it.
4) This beta will be slow and buggy. Release is still 4 months off!
5) You may have problems downgrading if you want to do so if you don’t have an SHSH stored etc.
In the meantime, remember you got all the new features already in your jailbreak:
1) 4.0 Multitasking = Install Circuitous / Backgrounder from Cydia.
2) Folders = Install categories / categoriessb out of Cydia
3) Music controls on home screen to control pandora or other = install music controls from cydia. Install backgrounder to background pandora.
4) Background wallpaper changing = Install winterboard or bosspaper and change your wallpaper is many more configurable ways.
5) Spellchecking = Inspell in cydia.