in the interests of making life easy for you and your clients, I know 2ndSite (http:// www.secondsite.biz ) just released a new version (early August) with a feature they call "direct link invoicing". It's pretty cool cause your clients can go directly to their invoice without logging in (formally)...I don't know about you, but some of my clients just can't get it together to log in using a password and everything so I'm digging this feature.
This release also included recurring invoicing (for paypal too) which is awesome...I'm still under 40 client accounts so it's still free. Pretty happy with 2ndSite myself...