You can use the UCSC mysql server:

mysql --user=genome --host=genome-mysql.cse.ucsc.edu -A -D hg18 -e 'select name,chrom,chromStart,chromEnd,observed from snp130 where name="rs35568883"'
+------------+-------+------------+----------+----------+
| name       | chrom | chromStart | chromEnd | observed |
+------------+-------+------------+----------+----------+
| rs35568883 | chr21 |   38782125 | 38782126 | A/G      |
+------------+-------+------------+----------+----------+

FYI, programmatic access to the UCSC database are also discussed in the following threads in BioStar.

• Get rs number based on position
• Is There Such A Thing As A Ucsc Api?

Or the UCSC table browser if you don't use mysql. http://genome.ucsc.edu/cgi-bin/hgTables?org=Human&db=hg18 Group -> Variation and repeats Track -> SNPs(130) Output Format -> Selected fields from primary and related tables. Then click 'get output' button. On the next screen check the boxes for name, chrom, chromStart, chromEnd and observed. Then click the 'get output' button on that screen.