I hate to see the gnome slip out of standard out-of-the-box D&D in 4E. But then again, the designers haven't really known what to do with the gnome for some time. They got the biggest revision of any PHB race during the 3.5 revision, and their niche seems to be vaguely defined as "part elf, part dwarf." Sure, they'll be in the MM, which means they'll get as much love as a PC race in future installments as the gnoll and the bugbear get in 3rd Edition.
And yes, as others have pointed out, having them appear in the MM but not the PHB means a player would have to get DM approval for using them, since they're not in the default PC races.
My first 3E character was a gnomish rogue/wizard. I made gnomes an integral part in my current campaign. My players don't really care for gnomes, but I dig them. I used my players' feelings about gnomes to help shape their racial identity, as IMC the other races hate the gnomes as much as most of my players do.
Of course, I've changed the story of the gnomes from what's in the PHB in order to make them more interesting and unique in my campaign: they were a fey race cast out of the faerie realm eons ago because of a prank pulled by Garl Glittergold -- a prank that went horribly wrong -- on the Queen of the Seelie Court. And boy does she hold a grudge.
Gnomes, in finding themselves new residents of the prime plane, created an advanced culture based on a blending of nature and magic. They love the pursuit of magical knowledge and dabble in magitech, and in their homeland there is a library of magical and natural history unsurpassed in all the world. But they always sought a way to return to the faerie realm. Then their homeland was destroyed half a millenia ago by hordes of kobolds and the black dragon they worship as a god.
Now the gnomish race drifts around in small insular groups rather gypsy-like, looking for a place where they will be welcomed. Except nobody likes them, nobody trusts them, and they can't seem to defend themselves very well. Now the gnomes are a dwindling race, on the brink of extinction, and where they have settled in the cities of men and dwarves they form tightknit little near-autonomous enclaves. They don't like outsiders because outsiders don't like them, and if one is in danger chances are the entire community will come to their aid.