I agree ... a solution from your cable company can be a very good interim solution. My cable company offers the Scientific Atlanta 8000 which I use and like alot. HD PVRS are starting to be rolled out by the cable companies too ... Scientific Atlanta 8000HD and Motorola 6208. These should certainly look great on a plasma! Rent it ... and then when something comes out that meets your needs ... buy it and turn the cable company box back in.
However, to your specific questions:
The LG LST-3410A is a DVR that includes QAM (digital cable) capabilities. However, it only allows for tuning unscrambled channels. Exactly what digital/HD channels your cable company is sending in the clear is, unfortunately, a bit fuzzy right now.
Hopefully at CES (Jan. 8 - 11) we'll see some product announcements ... I expect a cable plug-and-play DVR (you get a CableCARD from your cable company ... plug it in ... get all the channels you subscribe to) to be out sometime in 2004. However, I'd expect it later in the year rather than earlier. Again, we should get some news out of CES.
The current stand alone DVRs can certainly record from the cable box via composite or SVideo ... however, with that plasma this may not be the best solution for you. It won't allow for HD recording and you may see some degradation on digital channels.
By later this year (June I think, but I'm not sure) your cable company is supposed to be able to offer you a box with functional Firewire ports. Firewire should allow you to record digital channels (including HD) w/o degradation. Hopefully we'll be getting a clearer picture / more products for recording from a cable box's Firewire ports. Again, I'm hoping to see some announcements out of CES.
Unfortunately, right now Tivo basic is only available on the integrated DVD units. There are no stand alone simple Tivo units with Tivo Basic (existing or announced). Again, perhaps we'll see some soon ... perhaps not.
Some of the early Series 1 units allow for some very basic recording functionality w/o a subscription. However, I'm unsure of the exact capabilities they offer. You may want to check http://www.tivocommunity.com
for more information.
I know, not alot of great options right now. However, there are lots of people looking for the same thing you are and hopefully we'll start seeing some products (or at least product announcements) soon.