In browsers able to understand CSS normally, just define your background picture (radient or whatever) with a 1px border, as you always do :

input[type="submit"] {
background: url(“img/button.jpg”) repeat-x;
border: 1px solid #3B3B3B;
}

We know that if we leave it this way, IE7 will add an extra 1px coloured by the background color, which defaults to white. If we remove our own border, it will remove that extra pixel. So let’s think :

/** This is applied to IE7 only, with conditionnal comments **/
input[type="submit"] {
background-color: #3B3B3B; /** the color of our original border **/
border: 1px solid transparent; /** we have a border, so the extra pixel will stay, and the border is invisible **/
}

You’re welcome.