They will never sell a unit without a hard drive inside of it.
This is the solution for the immediate future. Long Long term... They do have some other plans, but for the practicle period of time... this is the eSATA feature.
Put on your crystal ball hat: do you think they will deliver a PC card capable of tuning D* under Windows Vista or multi-room viewing first?
I say this assuming that eSATA enhancements are likely to come after both. And both of those are germane to the issue of how much storage you have, each for different reasons.
While I'm at it, do you know what filesystem D* uses? I.E., could a PC read a D* eSATA drive?