New education law says that students must be in the "least restrictive environment". So if they can be successful in a mainstream classroom with support and assistance, that is judged to be the best place for them to be, rather than sequestered in a separate class. That assistance can be one-on-one help, or accommodations including extra time, speech-to-text software, preferential seating, or a huge number of other things.
Many students with more severe disabilities are still in separate classrooms, but there are even laws about how "separate" those classrooms can be. My previous school had a multi-age 8-1-1 classroom (8 students with various severe issues, 1 teacher, 1 TA) that was required, by law, to be in the same general vicinity as the other classrooms. It literally could not be down at the end of a hallway or separate from other classrooms. Those students came to music, art and most PE classes (some had special PE also due to physical impairments) with the rest of their age group peers, as well as recess and lunch. However, all of their academic work was completed in that separate classroom. When they entered middle school, they each went to a separate teacher for some subjects and had accommodations in others as they were able, again, to keep them from being segregated.
I think the idea behind the law is fantastic. I think many districts across the US do not always have funding, staffing or support to make mainstreaming work in all the situations where it could. Some non-special-ed classroom teachers do not have the training they should in order to be able to make it work, although teachers who have come out of school in the last ten years or so certainly should have more than those of us who started teaching in the pre-mainstreaming days, and it's a constant focus of professional development in just about any school - particularly in these days of standardized testing insanity. Any teacher in the field should have a long list of strategies to use with students with various issues, and people/resources they can go to for more ideas when they are stumped.
And trust me, it's not always the student with a diagnosis and an IEP that is the one taking up more of your time and attention than they really should...