Here's a raw proof-of-concept script written in ColdFusion that identifies & blocks fake Googlebots. This can be easily expanded to cache DNS responses & log new bots.

Verify_Googlebot.cfm

<cfscript>
/* based on info from http://googlewebmastercentral.blogspot.com/2006/09/how-to-verify-googlebot.html */
badBot = 0;
blockBadBots = 0;
ip = cgi.remote_addr;
userAgent = CGI.Http_User_Agent;

/* Sample  request values */
//userAgent = "Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)";
//ip = "179.179.65.180";  //bad
//ip = "66.249.67.29";  //good

/* Note: This script uses InetUtil.cfc and is available for download at
   http://www.phillnacelli.net/blog/index.cfm/2007/2/21/DNS-Lookup-in-ColdFusion  */

if (Findnocase("googlebot", userAgent) OR findnocase("Mediapartners-Google", userAgent)){
	inetUtils = createObject("component","InetUtils").init();
	try {
		dnsRecords = inetUtils.reverseDNS(ip);	// Perform Reverse DNS Lookup
		domainName = ArrayToList(dnsRecords);
		if (not refind("/*.googlebot.com$", domainName)){
			badBot = 1; /* bad; domain name must end w/"googlebot.com" */
		} else {
			javaInet = createObject("java","java.net.InetAddress");
			dnsLookup = javaInet.getByName(domainName);
			ipLookup = dnsLookup.getAddress();
			if (ipLookup neq ip){
				badBot = 1;  /* bad; domain name must resolve to IP address */
			}
		}
	}  catch (any e) {
		badBot = 0;
	}
}
writeoutput("<p><b>userAgent:</b> #userAgent#</p>");
writeoutput("<p><b>ip:</b> #ip#</p>");
if (isdefined("domainName")){
	writeoutput("<p><b>Reverse DNS:</b> #domainName#</p>");
}
if (isdefined("ipLookup")){
	writeoutput("<p><b>IP from DNS:</b> #ipLookup#</p>");
}
writeoutput("<p><b>badBot:</b> #YesNoFormat(badBot)#</p>");
</cfscript>

<cfif badBot and blockBadBots>
	<cfcontent type="text/html; charset=UTF-8"><cfheader statuscode="404" statustext="File Not Found"><CFABORT>
</cfif>