Can't Connect To Local User Instance

Just when I think I have this stuff figured out, it bites me back everytime.

I'm
trying to deploy an mdf out to a different machine, and attached to it with local user instance. I connect without problem from my development
machine with a local user instance, which also has Express Manager resident,
and I can connect to a server instance without problem.

What I can't do is
copy the .mdf database over to another workstation, which has Sql Native
Client installed, and connect to the database using a local user
instance! I can connect to a server instance without problem from this
other workstation. Here's my connect string for local user instance
that works fine from my dev machine:

Provider=SQLNCLI.1;Persist
Security Info=False;Integrated Security=SSPI;Data
Source=.sqlexpress;AttachDBFileName=<full path_name and mdf
filename>;User Instance=true;

The error I get upon trying to connect with other stations is:

Error -2147467259
Database not found or cound not connect to database

I've
tried this every way I know how. I've copied the database from my dev
machine both with and without having detached the database from the
local instance. I've tried detaching from a server instance and copying
the file over. Nothing works, and I've done this before! What gives with this thing?

Rick