Yes, have used disk2vhd to convert a live exchange server, it converted it without any interuptions and I was able to succesfully attach it in Hyper-V, activate server 2008 and all exchange services were running.
As you would already have more than ONE DC in your network, you should be ok.
Just make a note of the following :
Exchange Server's Network details :
Static IP
Subnet Mask
Default gateway
any other additional IP Addresses you have bind on your exchange box hardware.
Disk2VHD uses vss to snapshot and convert.
You would need to re-activate your server 2003 but that shouldn't effect as long as you get it activated.
You can always convert exchange / sql box to vhd and then attach it in hyper-v using a "internal" network so you can test if it is booting fine and then you can arrange a downtime, shutdown the real thing and bring your virtual box on.
Hope this helps