I have rooted a couple of EVO's but I don't have a clue why everything works the way it does if you know what I mean. Some of the people in these forums blow me away with how much they know and I honestly admire them and thank them for their efforts. You may want to double-check this recommendation before you do it, or somebody please put in their two bits as well before I brick Carlitos's phone.
Having that little disclaimer said, I was helping a friend reboot his new EVO the other night. I ran into this very same issue where I tried booting the phone into recovery and the phone vibrated multiple times and then wouldn't boot at all. However, I could power on the phone and it would stay at the HTC EVO 4G white screen without ever going anywhere.
The steps that I followed to resolve this issue are in the same root guide that I followed found
here. What I recommend to do is to download the hboot202-s_off-V71.zip that can be found at the top of the thread. Extract that file to another folder that can be easily located on your computer. After the files are extracted, go to the new folder that you just created and open the pc36img folder. From there, rename PC36IMG-MR.zip file to PC36IMG.zip. You will need to somehow get that file moved to the root of your phone's SD card. I had to use another EVO as a card reader but it worked like a champ. Either way, get that file moved over.
After you have copied the file over, with your phone off, press and hold the volume up key while pressing the power key. Continue to hold the volume up key until you get into bootloader. It should see the PC36IMG.zip file and load it. Don't worry if there are some errors on there. Select "Yes" when it asks you to update and then select "No" when it asks you to reboot.
From there, start at Step 10 in the tutorial that I linked you to and give it a shot. You will need to open a command prompt with the directory set as the new folder that you just extracted everything to. For example, if you extracted the files to a folder on your C drive called EVO, you will want your directory for the command prompt to be C:\EVO.
Start plugging away and see where you get. Good luck!