A couple of notes...
1. Something noticably missing from this thread is any talk about Spread Spectrum. I've lived in NYC and have no problems with my 900Mhz SS phone, and have no problems (no lost, interfered, or dropped calls. Zero. Zilch). I had some issues with a non-SS 900Mhz prior to that. The SS also provides significantly better security.
2. One of the things that steared me away from the 2.4 Panasonics was the fact that they were 2.4 from the base to the headset, but 900mhz ANALOG from the headset to base. This is totally unsecure. It always baffled me that they could push their phones as 2.4.
3. While it is true that 2.4 is generally a less crowded range than 900Mhz, if you remember back to your basic high school physics class, lower frequencies travel further. I haven't owned a 2.4 phone, but I would guess that while it may go through walls better, it probably won't have quite the range.
4. Having a cordless phone with a speakerphone in it is the ONLY way to fly. Once you have it you can't go back.
4. A last note on Panasonic cordless phones. They used to be great, but now they generally suck. Until someone proves otherwise, I will not buy another Panasonic phone. The phone I bought in 1987 still works (it's in my parents house hooked up to a 2nd phone line) after almost 10 years of constant use and the last 5 years of not so constant use. This phone survived college when I lived in a house with 5 guys who would throw the phone to each other like a football. Needless to say the phone took a lot of knocks and still keeps on going. I bought the same phone (at the same time) for a video store I worked in back in the day and it, too, got the crap knocked out of it but kept on chugging. The last few Panasonic I bought all died in less than a year. I went through 3 of them before I finally decided not to give them any more of my money. Someone at Panasonic really dropped the ball here.
Of course, your milage may vary.