Dorian,
I spoke with an FA on a transpacific flight about this; she said they're not allowed to take $$ tips from passenger for any kinds of favor they did, simply because it's their job. (of course, some pax would abuse this privillege but..that's another thread

)
On DL or UA, there used to be this small booklet of coupons (someone remind me what they're called...) that they sent out to the frequent flyer members, which you could hand out to an FA for his/her exceptional service. They (the FAs) usually would react very positively to this, so you could treat this like your money tips.
The FA can then fill out this coupon with his/her employee ID and then forward it to HR/Corporate. Perhaps then it would be duly noted in their personnel records.
That's my $0.02.

Thx
Jamester
P.S. : The coupons have been discontinued since earlier this year.
[This message has been edited by Jamester (edited 11-09-2000).]