As for the Hammerhead, it's basically the Mako of ME2 from what I've read. And you're right about there being no missions where you could use it, but I imagine that'll be part of the DLC package.
Don't forget most of the planets you can explore in ME1 are basically just randomly generated terrain. I doubt it'd take much effort to generate a handful of terrain maps and attach them to planets which have no significance except scanning at the moment.
I can only hope they make some planets landable without being N7 missions... or else after you've completed the few missions that will be bundled with the Hammerhead release, there's pretty much no use for the content - unlike the current DLC content.
Oh and if they do sling more missions in, then they may increase the level cap, maybe? If you play all the N7 missions (check here for a list of missions Link)in the pre-DLC game - along with all the storyling missions - it is possible to hit the top level of 30... so maybe expanding it to the ME1 cap of level 40 might be a good thing too. More rewards in the beginning of ME3!
We'll see!
[Edited by MysticalNomad, 2/21/2010 3:17:00 AM]
[Edited by MysticalNomad, 2/21/2010 3:17:47 AM]