can i let stuff on the ground and come back to pick them later , like the house in fallout 3 ??

any basic map you can, special maps (combat, random encounters etc) seem to be generated on demand & destroyed afterwards so anything you need to take from them, do it right away.
If you're concerned about leaving good items that the opposition "donated" behind, you can usually come back to pick through the bodies later.
You'd probably be best off finding a bookshelf or container of some kind thats in a memorable place and use that for long term storage if you need it.

If you ment Fallout 3, it seems there isn't a totally clear answer to that question, yet. I am very certain that things you drop in whe Wasteland will vanish after a while. But for example I was able to drop some pieces of armour at a place, fast-travel to Megaton, and when I came back it was still there.
Whatever you drop in enclosed spaces like the city of Megaton or Rivet City, or especially your house, should remain there infinitely. I have heard that some people lost a few things they kept in their house, but I suppose that rather happened due to a bug. It didn't ever happen to me in the 93 hours that I've played Fallout 3 :)
So, short answer: As long as you don't fall victim to a rare bug, your possessions should be very safe in your own house, supposedly still safe in other enclosed spaces, and only safe for short periods of time in the wasteland.

They DECAY?
Nah, I don't think so, man.

I didn't think so either, but on another board somewhere within this forum, that was the general consensus, as the bodies decay, so do the items.

I think he means Fallout 1, 2 and Tactics.
You can drop stuff on Fallout 1 and 2 and pick up later as long as you're not outdoors (ie. travelling), in Tactics I don't know.
The bodies do disappear after some time (leaving just the blood), and I think the items they have might disappear with them too. But if you drop any item pretty much anywhere, I'm pretty sure it stays there forever.
I think I always picked up every goddamn little piece of rubbish anyway. :)

Ah, right, I misunderstood the "like in Fallout 3".
I've experienced that after the bodies disappear, all the items they carried are left lying on the ground. For example I could come back to the first raider camp of Fallout 2 near to the game's end, and all the weapons and armor they used would still lie on the ground, even though the bodies were long gone.
So, I think it's safe to say no item ever vanishes completely in Fallout 1/2 if dropped somewhere, unless it's a place you won't have access to anymore, like the random encounters.

F1 and F2 saves every location you visit on leaving it (take a look on space usage of your savegame ;) ) items including, so you can left it on the ground. I am not sure about special encounters, at least at FOT those encounters was saving items on the ground in another way, so you were able to use them as infinite supply (it saves location vhen first left and then loads this save, look on google for more info)
Only problem could be blood in F1,2 it can hide dropped items.

In FO2, I've noticed that even the blood goes away after a while, and this makes it easy to pick through the loot for the stuff you want without having to open the inventory screen.
Apparently, if you are humping and impregnating all of the female relatives of a certain casino owner while ignoring his job offers, he gets cranky.
Words were exchanged, guns were drawn, bullets flew. Lots of bullets.
I can understand the guards shooting at me after I turned their boss into bacon mist. Not sure why all the Casino customers and dealers thought it would be a good idea to come after power armored me with knives and shivs.
Anyway, there were only three survivors: wife, daughter, and a random prostitute.
Everyone else eventually became a convenient little pile of goodies. You could mouse over each pile and quickly figure out what was there and if you wanted any of it. I took all the money, just for shits and giggles.