You will not get a full years aid for attending for one semester. You will just get a semester's worth. Financial aid is usually calculated on an annual basis and disbursed on a semester basis (or quarter for schools on quarter systems). For instance if your Pell eligiibility for a year is $4000, you would get only $2000 for the spring semester (though you might be eligible for the balance in the summer if you took summer classes). Not having gone to school in the fall would not mean you get the whole year's Pell in the Spring semester. Loans may be handled differently. When I started school in a Spring semester I did get more than half my annual loan eligibility in the one semester.
Spring aid might
be lower than aid offered in the fall. Some programs, such as SEOG and WS, have very limited funding and there is unlikely to be any money left in those programs to offer in the Spring (those funds at our school for the 2011-2012 school year were fully awarded by feb 2011). Some scholarships may also not be available to students starting in the spring (that would depend on the school and the scholarships)
(I am assuming that is the case because I know that the new FAFSA begins on January 1st).
The new FAFSA that begins on January 1st is for the 2012-2013 school year that starts in the fall (or summer for some schools) of 2012. Spring 2012 is part of the 2011-2012 school year and will use the current FAFSA. You will be offered aid that covers just the Spring semester.