I completely agree with this. I signed up for Google Apps for the sole purpose of having an email address at my own domain, and I always wished I could just have that email address supported by my existing Gmail account, rather than having to deal with the hassle of yet another account somewhere. I would gladly have paid for that.
Google's always been pretty good about making sure you aren't locked in to their services, likely due to the efforts of their Data Liberation Front[1], but it always seemed to me that being locked in to using an @gmail.com email address was a big exception to this.