How / is it possible to compare files on your hard drive with those in your time machine backup?
I realize you tagged this as a Snow Leopard question, but if you upgrade to Lion, the new tmutil command-line utility will let you compare backups:

Usage: tmutil compare [-a@esmugtdrvX] [-D depth]
       tmutil compare [-a@esmugtdrvX] [-D depth] snapshot_path
       tmutil compare [-a@esmugtdrvX] [-D depth] path1 path2

Perform a diff of or against backups.

In the first form, tmutil will compare the computer to the latest snapshot.

In the second form, tmutil will compare the computer to the specified snapshot.

In the third form, you can compare arbitrary things to each other, including things on the computer to things inside your backups.

The compare verb allows you to specify what properties to compare. The default property set is equivalent to the -smugt option set. Specifying any property option overrides the default set.

# this means "compare using the basic -smugt metadata set"
$ tmutil compare

# this means "compare all metadata supported by tmutil and data forks"
$ tmutil compare -ad

# this means "compare extended attributes, ACLs, file modes, and data forks"
$ tmutil compare -@emd

Options:
    -a   compare all supported metadata
    -@   compare extended attributes
    -e   compare ACLs
    -s   compare sizes
    -m   compare file modes
    -u   compare UIDs
    -g   compare GIDs
    -t   compare mod times
    -d   compare file data forks
    -r   ignore specified root paths and just compare descendents
    -D &lt;depth&gt;   limit traversal depth to &lt;depth&gt; levels from the beginning of iteration
    -E   don't take exclusions into account when comparing items inside volumes
    -v   enable verbose mode (enables reporting of all identical items, which is noisy)
    -X   print output in XML property list format