i would recommend you use a combination of POS tagging and then string tokenizing to extract all the nouns out of each abstract.. then use some sort of dictionary/hash to count the frequency of each of these nouns and then outputting the N most prolific nouns.. combining that with some other intelligent filtering mechanisms should do reasonably well in giving you the important keywords from the abstract
for POS tagging check out the POS tagger at http://nlp.stanford.edu/software/index.shtml
However, if you are expecting a lot of multi-word terms in your corpus.. instead of extracting just nouns, you could take the most prolific n-grams for n=2 to 4
