There are ways you can keep your current MX records pointed at your mail server and then pass mail onto the Google services. To do this you register your primary domain. Then verify you own the domain using a cname or HTML file. Once you have proven ownership the system will let you send email out as that top level domain, even if the MX records are not pointed at the Google servers.

Then, you create a domain alias (possibly with a sub-domain.) You then set the MX records of this alias to the Google Apps servers. Now you can either set your current system to forward mail also to this alias, or use MTA routing rules to route to the Google servers they list to point your MX records using the alias. I know this works, and if you really want to use both Google Apps and another system, this is the way to do it.

Best of luck!