CVJ,

Thanks for the reply.  Are the 18 character IDs new, or is this something I just hadn't run into previously? 

I've just had to implement the logic for converting 15 char to 18 char identifiers as described by cvj in JavaScript. I thought i'd post it here in case anyone else happens to find this useful...

function convertToCaseInsensitiveId(id) {
    var hash = '';
    for (var c = 0; c < 3; c++) {
        var h = 0;
        for (var i = 0; i < 5; i++) {
            var ch = id.charCodeAt(i + c * 5);
            if (ch >= 65 && ch <= 91) h += Math.pow(2, i);
        }
        hash += String.fromCharCode(h > 26 ? h + 48 - 26 : h + 65);
    }
    return id + hash;
}

David's Javascript is very useful because you can port that over to a Google Spreadsheet and use it like a "macro." 

1. Open a Google Spreadsheet
2. Select Tools > Scripts > Script Editor
3. Paste David's function
4. Save and exit the Google Apps Script Window
5. Go back to the spreadsheet. Type a 15 digit salesforce ID in Cell A2
6. In another spreadsheet cell, type = convertToCaseInsensitiveId(A2)

A programmer named Stefan Kuehlechner made two updates to the David Padbury script (in bold).

function convertToCaseInsensitiveId(id) {
    var hash = '';
    for (var c = 0; c < 3; c++) {
        var h = 0;
        for (var i = 0; i < 5; i++) {
            var ch = id.charCodeAt(i + c * 5);
            if (ch >= 65 && ch < 91) h += Math.pow(2, i);
        }
        hash += String.fromCharCode(h > 25 ? h + 48 - 26 : h + 65);
    }
    return id + hash;
}

I hired a programmer to write the david_padburry script as a free, open source Google Spreadsheet script that is much easier to implement.

Here is an article that describes how to install and use the 15 to 18 digit Salesforce ID Converter from the open source Google Spreadsheet Script Gallery.