lookupZIP <- function(city = "Iowa City", state = "IA") {
    require(XML)
    
    if (length(city) != 1 || length(state) != 1)
        stop("can only handle one query at a time")
    
    city <- gsub(" ", "+", city)
    
    queryURL <- paste("http://www.zipinfo.com/cgi-local/zipsrch.exe?ll=ll&zip=",
                      city, "+", state, "&Go=Go", sep = "")

    ## Parse the query using capture.output to suppress output.
    capture.output(z <- htmlParse(queryURL))

    ## When a single matching city is found the response contains a
    ## table of data with a header row (numCol th elements within a tr).
    ## This is the table we want. If there is no such table then the
    ## Find call returns NULL.
    numCol <- 5
    tableHasHeaderRow <- function(x) length(getNodeSet(x, "tr/th")) == numCol
    dataTable <- Find(tableHasHeaderRow, getNodeSet(z,"//table"))
    
    if (! is.null(dataTable)) {
        ## Drop off the header row.
        dataRows <- getNodeSet(dataTable, "tr")[-1]

        ## Extract data values, using as.character to remove names.
        extract <- function(r) as.character(sapply(xmlChildren(r),xmlValue))
        rows <- lapply(dataRows, extract)

        ## Form result into a data frame with appropriate variable types.
        val <- do.call(rbind, rows)
        rownames(val) <- NULL
        data.frame(ZIP = as.integer(val[,1]),
                   city = val[,2], state = val[,3],
                   lat = as.double(val[,4]), long = as.double(val[,5]))
    }
    else NULL
}