Thank you for two excellent questions. First, “Do all cells in a Bt-containing transgenic plant contain the Bt transgene? The general answer is yes. Barring rare mutation or rare chromosomal abnormalities, all somatic cells in a plant contain the same DNA. The large part of a cereal seed (corn, wheat, oats, rye, rice, etc) is called an endosperm and it has three doses of each gene whereas other cells and tissues contain only two copies. However it is the same complement of genes in all tissues. Germ cells (pollen and ovaries) contain only one copy of each gene and these cells can differ genetically if the plant producing these cells contains two different types or forms of a particular gene. This would be the situation in plants that exploit hybrid vigor, such as maize or corn.
What is true in plants is also true in animals. Interestingly, your nose and your tongue have exactly the same set of genes, yet the tissues do different things and are different in size and shape. The reason for this is differential gene expression. While the tissues have the same genes, different sets of genes are expressed or turned on in the different tissues. An integral factor underlying this differential gene expression network is a part of the gene called a “promoter”. This is a DNA sequence that binds various proteins and other molecules that are expressed in only certain cell types. This sequence “says” express in the nose, not in the tongue, in the liver, but not in the heart, etc.
The same system operates in plants. Many of these DNA sequences (promoters) have been identified in plants and this allows investigators to express the coding region of a gene of interest (for example the coding region encoding the Bt protein or the RoundUp ready protein) in particular tissues.
So transgenes usually contain parts derived then from different sources. If investigators wish to express a gene say only in the cereal endosperm, they would employ a promoter that was shown from previous experiments to be expressed only in the endosperm. Once transgenic events are isolated, the investigators would then check to make sure the gene is expressed in only those tissues in which they want expression. Levels of expression would also be monitored since promoters can be distinguished not only by the types of tissues in which they are expressed but also by the amount of expression.
So, there is no one answer to your second question about the tissues that express the Bt protein. It depends on the promoter and the particular transgenic event under study. Given that most tissues of a plant are susceptible to insect attack, promoters expressed in lots of plant tissues would be favored.