Actually, Bitt, I've been reading over the rebate a couple times, just to make sure I wasn't getting screwed over or anything. It appears that the requirements are:

1) Order the Series 3 by June 16th
2) Activate a Tivo service by July 16th
3) Send in the rebate postmarked by August 16th (but also a month after your Tivo service is activated)

So you have a month to activate the service. I'm pretty certain you can do that without actually having cable, since Tivo doesn't know what service you have (or really care, as long as you're paying them).

I will be out of town from July 1st to July 16th (my birthday!), so I'm going to have to activate the Tivo service before I'm signed up for FIOS or even living in my new house!

*edit*
Oh, and congrats! I follow your thinking and find it perfectly reasonable. I had to run my S3 purchase by the fiancee, as we're a little low on cash, but my reasoning was that the Series 3 will not get down to this price again in the foreseeable future, and I was going to have a yard sale to pay for the $600 Tivo anyway, so I'm actually saving us money. Now that I'm getting married I'm going to have to really practice my justifications for new technology. Previously I just had to justify it to myself, and I'm a real pushover when I ask myself if it's okay to get a new gadget

*edit 2*
By the way, if you're interested in expanding the recording capacity of that new toy you just bought, I have a recommendation: this external drive enclosure. Check out the results they got in this review. It does an amazing job at cooling, and it's pretty quiet. I ordered one for my parents' HR20, and we couldn't hear it.

That's another story, though. It turns out that yes, the eSATA port on the HR20 is enabled. The problem? It's an either-or proposition. Either you use the external drive or the internal one. So if we wanted to use the 750GB drive we ordered, we'd have to lose all the programs we'd recorded already. My parents weren't pleased by that idea, so they're going to hold off and hope that a software update takes care of that problem and allows a simple addition of storage capacity.

Apparently the Tivo merely adds the new space to what you have already. You just have to follow those instructions I posted somewhere here on the board.