I think there is a combination of factors at work here. First, 1E AD&D did have a number of save or die situations, or instant death, no save even. Those have been removed in 3.x. CR's also provide some guidelines as to appropriate challenges.
The next factor is the number of options open to characters. This has increased survivability and also made sure that if a DM does not build adversaries well, then the mortality rates have gone way down. So, the DM has to know more rules if he allows all of the options open to players (or even most of them). Also, I think the power spiral started earlier and steeper in 3.x. By this I mean that in order to increase sales of supplements, there has to be some upward creep in power to make neater, more attractive splatbooks that players will want to buy. With production of adventures left to 3rd party publishers, WOTC is selling mostly to players, thus the power spiral has been accelerated. I do think it could be worse (2E's was pretty steep). Fortunately, the DM has access to these supplements and can ban his players from using them to build characters.
The third factor is a change in role-playing style. I have done a bit of research on the differences in gameplay between different age groups (reading of existing material, informal surveys, etc.) over the last few months. It has become intriguing to me (and Monte started it with an article in Dungeon that got me thinking about it, plus it became an issue at my table). During the game's formative years (OD&D, BD&D, then 1E AD&D for a bit), there was not as much emphasis on story. As the storyteller style was developed, it became more important to keep characters alive for the continuation of story and plot. Thus, lethality of adversaries was lessened to allow for good role-playing and good drama (not that some early players didn't start this). I think some of this has translated to the DM'ing style of many DM's that started playing the game from the late 80's until now. Monte actually does a pretty good job of explaining it (old school v. new school DM'ing) and I believe it. I have seen it at work. Also, players expect to live more and develop cool long-term characters (not that we didn't have them in the early 80's, they were just more disposable, since they were less customizable), and since most DM's need happy players to continue DM'ing, they have allowed for this long-term character development, to get PC's into the PrC's and feats that are available.
All of that being said, I am an old school DM. My first 3E campaign, I was learning the new system and made resurrection magic freely available, even at relatively low levels, because I wanted to see how the new system worked as it progressed. After that (the PC's reached about lvl 17), my campaigns have been notoriously deadly, even though the continuing plot elements are present. My second 3E campaign reached lvl 18, but there were probably 2 dozen character deaths in there (one player accounted for pretty much half of those and the rest probably lost 2-3 characters each on average), including 1 TPK. My next campaign died due to player problems and lack of continuity due to high mortality rate (about 6 PC deaths in three months). Then we played D20 Modern and there were few deaths (we played to about lvl 12) and went back to D&D via Midnight with the Grim & Gritty rules, where the mortality rate was actually not as high as one would think (IIRC, 3 PC deaths in about 6 months). After Midnight, we played a short sea-going campaign with a high mortality rate (both of my campaigns with the highest mortality rate were sea-going, hmmmmm.... I think it is the swashbuckling bravado that makes them foolish). My current campaign has had 2 PC deaths in about a dozen sessions, both from the same player (the guy who racked up a dozen in the campaign mentioned above - he is not stupid, just doesn't like to run away - he was also responsible for most of the deaths in the Midnight game for the same reason). So, byond the above mentioned combination of factors, I think it depends on the DM, the game style and the players.
Just my 2 CP.
DM