Comment Targeting is the big problem (Score 4, Informative) 97
Talking about cancer as "a disease" is a big misnomer; at best it's a huge family of diseases (really nice explanation in this comic). Patterns do emerge -- certain tissues tend to have similar patterns of gene expression between people and therefore tend to give rise to similar cancers -- but each cancer that arises comes about in a different way, and evolves in response to different selective pressures within the body. The biggest of these pressures are fairly obvious like the need for neutrients (so "successful" cancers are the ones that evolve the ability to encourage blood vessels to grow around them) and evading the immune system. So, almost by definition, the outside of a cancer cell is forced to look as similar as possible to the outside or a healthy cell in the same tissue, to avoid detection.
There are some exploitable internal differences. Most cancers (but by no means all, or even close to all) express hTERT, a gene responsible for repairing the telomeres, whose degradation would otherwise limit the cells' replication. So some researchers (including my former lab) are working on techniques to exploit that e.g. viruses that can only kill cells expressing hTERT. The downside is that some legitimate cells also express hTERT, most notably your stem cells (bone marrow, some other tissues).
Another popular method is just targeting all cells that are highly metabolically active. Cancer cells tend to be working unusually hard (most cells in your body just sit there gently ticking over most of the time), so some cancer therapies target any cells that are burning through a lot of glucose (e.g. radiolabelled glucose is used as a source for imaging techniques like Positron Emission Tomography) or that are doing a lot of DNA replication as part of cell division. Again, though, this targets many cells in your body which are working this hard as a normal part of their programmes.
So, yeah, this is a cool payload but targeting is the hard part. If we knew what ligands to tie these particles to for targeting and how to persuade these huge particles to move against a pressure gradient and through a dense, disorganised extra-cellular matrix, cancers wouldn't be half the problem that they actually are. We could be using targeted viruses (piece of piss to do if you know what you're targeting and the surrounding tissue isn't too dense), metal nanoparticles, targeted liposomes (little hollow balls of fat) containing toxins or toxin precursors, modified antibodies to alert the immune system to the cancer cells, etc, etc.
Curing a cancer would be pretty easy: throw enough researchers and resources at one patient's specific tumour and we'll come up with a damn fine treatment. But curing all cancers -- different tumours arising from different tissues in different patients -- is seriously hard. We'll see fantastic advances in treating specific cancer types, but I seriously doubt that "a cure for cancer" is possible within our lifetimes. Although, heh, if you prove me wrong I won't be too upset