In many cases , the Cydia application fails to install or update apps after jailbreaking your Iphone with v3.0. There seems to be a conflict with the ICY and Cydia Apps installed on the jailbreak.

To fix:

1.       Backup your phone

2.       Re-jailbreak your phone using redsnow.

3.       Deselect Cydia,

4.       If you’d like ICY installed select ICY

5.       When the jailbreak completes, you should have a working install of ICY and Cydia.

Now that I've had the 3.0 firmware for a week, I'd like to share my first impressions.
I find 3.0 to be more stable than the 2.X versions. My Iphone no longer reboots itself throughout the day.
I love the cut and paste and new search capabilities as well as the tether. I also like the landscape keyboard, I no longer have to "fat finger" the tiny keyboard keys.
I love the support for my Bluetooth Stereo Headset for listening to Itunes, other audio apps, and taking phone calls.
I haven't much need for the voice recorder, though I Imagine others will.
What I don't like: Many of my Cydia apps will not install or update due to dependency issues. Looks like I might have to uninstall them and start over.
All in all increased stability, nice new features, doesn't play so nicely with Cydia.
Are you running 3.0? what do you think?