While there have been some improvements to Bluetooth support through the version – like the addition of A2DP (Wireless Stereo) and most recently PAN in AKU3, at the same time the Dial Up Networking profile has been removed. This is a very widely supported profile for connectivity sharing. If you previously used your Windows Mobile device to share your GPRS connection you’ll no longer be able to do this with newer devices running AKU3 (or perhaps if any OEMs offer ROM updates to AKU3 for current devices). One scenario I’ve already encountered where this is a problem is using a TomTom ONE/Go unit – these use Bluetooth to use your phones connectivity to download traffic updates etc, and they support only the DUN profile.
Read a discussion about these changes here on Smartphone Thoughts.