Definitely issues with Apples new SMB implementation. I have managed to get SMB working and will try to explain what I think is happening.
As best as I can tell, when we try to map a SMB or CIFS share from the GUI, it appears to send the username and password of the logged in user. It appears to completely ignore any username you add to the url.
The ugly work around (Mount from the CLI)
Fortunately for us, there is the CLI method. You will need to open terminal for this.
First we need to make a place to mount the share to. When you first open terminal, it should place you in your home directory.
Lets make a folder called "data" by entering the following:
mkdir data
In my case, this will have created a folder in my home directory named "data". Now that we have a folder to mount the share to, lets get down to business. Below is the command used to mount a share. I'll explain it:
/sbin/mount_smbfs //myusername@myserver/share data
myusername is the username you need to connect with.
myserver is the server name or IP of the server you wish to connect to.
share is the share name you wish to connect to.
data is the folder you wish to mount this share to, in this case we created "data" as the mount point.
Give this a little time after hitting enter, it will prompt you in the terminal to enter your share's password. As you type, you will not see anything on the screen, but it is taking the password. Once you've completed entering your password it will mount to the folder you specified. You can access your files from in this folder, or if you have set Finder to show mounted shares on the desktop, you will see it on the desktop as well.
The only other odd thing here, is it does seem to have issues with then trying to mount another share from the same server with the same username. It has allowed me to mount other shares with another username on the same server. Really odd.
Not an elegant work around, but it at least gets us to our files. Here's to hoping someone at Apple eventually answers.