I've been wanting to ask other Beatles' fans about this for a while. I am sure I can hear Yoko singing in these songs: All Together Now (though she wasn't around the Beatles' in '67, was she?), and the 'love you' part on The End. I hope I am wrong! Does anyone have any info on this?

I think she's singing in Bungalow Bill for sure, but I hate that un-Beatles song, so I don't care about that. But, as for the others, can anyone shed any light?

yeh she's singing on bungalow bill, rev 9 and some other one off the white album...also u can hear her singing(wailing?!) at the end of the studio version of dont let me down

The first I heard about it.  
I'm not sure (noone can really be, too many years, y'know...   ) but I think NO, NO, NO AT ALL.(IMO) Only in Bungalow Bill, but no in the other two.  
There are a lot of sources you can consult, just name three:  

http://dmbeatles.com/lyrics.php?category=1
http://www.upv.es/~ecabrera/
http://www.beatles-discography.com/

Tho I don't trust too much in any (not just these three but anyONE) because they used to copy one each other and have many mistakes (every fan can point them)  
In cases like this I usually listen carefully to the song once, twice,... a lot of times till I can get an idea (I've need hundreds of hours of carefully listening of their songs to know -somehow- the way they played, sang or wrote songs: really hard, man!!).

She's in the choir with Pattie Harrison on Birthday.

Of course she was (and in Revolution 9, and I don't remember -know- now but maybe in another one or two apart "Give Peace A Chance" (Lennon/McCartney?)), but in the three ones above only in Bungalow Bill (I think).

I'm sure I can hear a female voice in the 'love you' part in The End. It spoils my enjoyment of it just a tiny bit sometimes. If it's a woman singing, it must be Yoko I suppose.

Quoted from Bobber

She's in the choir with Pattie Harrison on Birthday.

I thought they brought in some of the Apple Scruffs for that one.

Quoted from juniorsfarm

I thought they brought in some of the Apple Scruffs for that one.

That's what I heard too. I'm pretty sure that's what it says in Mark Lewisohn's book.

DM's site says it's Pattie and Yoko. I've read that elsewhere too. Ah! A new mystery!

It's the same old sotry: Who's playing bass in Drive My Car? Everyone seems to say it's Paul but George Harrison said in Anthology Book it's him and Paul's playing guitar. There are lots of examples. IMO Pattie and Yoko sang in Birthday and two Apple Scruffs (brazilian Lizzie Bravo and his London friend Gayleen Peasesang) in Across The Universwe (and nowhere else).