The big thing here is BioWare's statement(s) that being unfaithful in ME2 will have consequences in ME3, and that it's possible to cheat on the ME1 LIs. This sells the idea that, no matter what we did or did not do in ME2, the ME1 LIs will be in ME3. It's possible to get Garrus and Tali killed, but it's easy enough to rollback your game and obtain any outcome you want have to bring into ME3.
As stated above, Liara's role in ME2 was to ensure Shepard was actually in the game (or should I say story?) at all. And she paid for it, and now has to hunt the Shadow Broker. Kaidan and Ashley fans gets less juice than that, as their Shepard's are railroaded into Cerberus and don't get the option to jack it all in and follow their lovers back to the Alliance. They're stuck (at best) with a hopeful e-mail and a photo. Their lovers don't have a substancial story hook to bring to ME3. They went right back where they came from, back to the Alliance, and fall under "pending".
But big things are moving in the background. The geth can get stronger and find unity. The krogan might get a cure. The quarians might try to wage war on the geth (no matter what you do, actually). The rachni might be out there, in the wings, ready to aid against the Reapers. Dark energy is floating about, rather ominously. And so on, down the line. None of these aspects got major "air time", but were slipped in there, setting things up for the final act.
What we are, therefore, expecting is that BioWare's writers have some idea how all this will come together and get addressed to a suitable and satisfying conclusion in ME3. This includes the LIs from the original game. Granted, most often, when you invent something new, you don't make massive plans for a sequel, so ME1 can stand alone. But ME2 is definately the middle, with ME3 mentioned and planned for. So, no matter what the fanbase is for Tali, Liara, the biotic god or that fraking hampster, these story threads are going to come back. And, knowing BioWare, they might well slap us in the face. In fact, they already did that with Liara, Kaidan and Ashley, so we know they are quite capable of doing that. Plus, as Stanley Woo has pointed out, they can do so whether we like it or not. They will be making the choice, in the end, not us. They can wipe out Tali, Garrus, Thane, or whoever, just as easily as we can in ME2.
No matter what the fanbase is doing by the time ME3 is completed, the one certainty is that Shepard, Kaidan, Ashley and Liara will be there. It's up to you to decide who else makes it, and it's up to BioWare to work out where all these story threads and choices will end up. Have your save games ready, it's gonna be one heck of a ride!