Colony collapse in bees was observed and reported before the introduction of GM crops. Also, it is occuring in Europe, where the production of GM crops is miniscule.
There were some pretty serious methodological problems with the Bt corn residue study, foremost among which was the failure to verify that Bt corn was actually grown in the watersheds studied. Note that Bacillus thuringiensis is a ubiquitous, naturally-occuring soil bacterium. Bt extracts (including live Bt sprays) have been used in organic agriculture for many decades and resistance to Bt arose in some pests long before the release of the first Bt-engineered crops. Any time an intense selective pressure is applied to nature, results will show up fairly quickly. The resistance that Glassman reports have been around for years and seem peculiar to certain specific field/crop conditions; most notably, the apparent resistance hasn't progressed (spread) from when it was first observed...a very unusual and unexpected result, given the widespread use of Bt crops. Similar observations have been reported for Bt cotton by Tabashnik (I think).
The issue of messing with nature is a tricky one, since agriculture itself has had the greatest adverse influence on the world's ecosystems...and long before the introduction of GM crops. FarmsWithJunk is correct to point out that, except for some wild fruit/berries and fish& game pretty much all our foodstuffs have been intensively genetically modified over many generations. He is also correct in saying that modern gene-splicing techniques are radically more accurate in improving crop charateristics than are other methods.
A typical GMO will have a "cassette" of several genes introduced into it using one of several methods (biolistics, agrobacterium-mediated insertion, etc.). To many folks, this sounds really unnatural (and maybe unsafe!), but a very large research study in the EU on the issue concluded that this technology was likely to be as safe as conventional breeding, and might be safer in some cases.
There are some pretty intrusive techniques included in "conventional breeding"...eg, mutation breeding treats seeds with ionizing radiation or mutagenic chemicals to induce random mutations, which are then screened for desirable changes (the problem is that many background mutations are never tracked at all)[Clearfield herbicide tolerant canola and some varieties of pink grapefruit were made this way] , wide crosses involve forced crossing of species that would not normally interbreed successfully in the nature (often using embryo rescue to get a viable plant). Agriculture and the plants in that system are not "natural". Most of our crop plants are dependent on humans for their survival; they would not last more than a few generations (if that) competing in the wild.
How about cancer rates? Worldwide, some cancer rates are increasing and others decreasing; this varies from country to country. Cancer stats always have to be age-corrected, too. I didn't have a comparison graph for colon cancer incidence handy, but here is a reference for overall cancers (note that several high-ranked countries are in areas where GMOs are effectively banned:
Cancer rates: see how countries compare worldwide | News | guardian.co.uk
Pretty much everyone has big areas of ignorance. This is natural, after all, you don't really need to know how your cell phone works to be able to use it, nor do you need to know the genetic history of your lettuce or tomato to enjoy your salad. However, ignorance is a fertile field for misinformationists. It is easy for somebody to pull the wool over my eyes about areas I know little or nothing about. Every day we get another health scare (or three!); in a society of surplus, we have the luxury of spending the time to fret about our food, our medicines, our air and water quality. If you enjoy the conspiracy theories, or the frisson of disquiet from these scares (or for anyone else who wants an overview of the whole gamut of anti-GM arguments), pick up pretty much any book by Jeffrey Smith or visit a GreenPeace website
Say no to genetic engineering | Greenpeace International , GMWatch
GMWatch or any of a plethora of other (usually) interlinked sites. There is a parallel universe of sites presenting the other side of the coin (and usually spending a lot of time showing why the other stuff is junk science!), like David Tribe's GMOPundit site
GMO Pundit a.k.a. David Tribe or Dr. Prakash's AgBioWorld site
AgBioWorld - Supporting Biotechnology in Agriculture. It quickly becomes a he-said/she-said kind of tennis game and each reader has to decide who has the most credibility.
BOB