(This post is mostly for the search engines, to help others looking for the solution that this post provides)

Lightroom, Adobe's high-volume photo workflow application, stores all kinds of user-preference and per-image data in an SQLite database. As of the Beta 3 release, the database is stored in one file, the Mac filename being “/Users/username/Pictures/Lightroom/Lightroom B3 Library.aglib”

If you're technically-minded and want to peek at the database directly, you might try to access it with the sqlite3 command, which comes standard with OSX, and be disappointed to find that it doesn't work:

$ sqlite3 'Lightroom B3 Library.aglib'
SQLite version 3.1.3
Enter ".help" for instructions
sqlite> .schema
Error: unsupported file format

Eric Scouten provided the answer in this thread on the Adobe's Lightroom Beta discussion forum. It turns out that Lightroom uses features of SQLite introduced in its version 3.2. OSX (as of 10.4.7) ships with SQLite version 3.1.3, and so it can't read Lightroom's database.

If you have Apple's XCode tools installed, you can download the source from sqlite.org and build a more recent version (3.3.6 is the latest as of this writing), and then it all works:

sqlite3 'Lightroom B3 Library.aglib'
SQLite version 3.3.6
Enter “.help” for instructions
sqlite> .schema
CREATE TABLE Adobe_AdditionalMetadata (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    image INTEGER,
    metadataPresetID,
    xmp
);
CREATE TABLE Adobe_FTPPresets (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    passive,
    password,
    path,
    port,
    protocol,
    server,
    title,
    username
);
CREATE TABLE Adobe_cameraRawSettings (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    name,
    rawSettings UNIQUE
);
CREATE TABLE Adobe_imageDevelopSettings (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    croppedHeight,
    croppedWidth,
    digest,
    fileHeight,
    fileWidth,
    grayscale INTEGER,
    historySettingsID,
    image INTEGER,
    settingsID,
    text,
    whiteBalance
);
CREATE TABLE Adobe_imageFiles (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    absolutePath NOT NULL DEFAULT '',
    idx_filename NOT NULL DEFAULT '',
    image INTEGER NOT NULL DEFAULT 0,
    importHash,
    lc_idx_filename NOT NULL DEFAULT '',
    markedMissing,
    md5,
    modTime,
    relativePath NOT NULL DEFAULT '',
    robustRepresentation
);
CREATE TABLE Adobe_imageProperties (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    image INTEGER,
    propertiesString
);
CREATE TABLE Adobe_images (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    aspectRatioCache,
    captureTime,
    developSettingsIDCache,
    fileHeight,
    fileWidth,
    hidden INTEGER NOT NULL DEFAULT 0,
    importedTime,
    orientation,
    originalCaptureTime,
    originalRootEntity INTEGER,
    panningDistanceH,
    panningDistanceV,
    photoBinNominators INTEGER,
    photoBinPosition,
    printSharpening,
    propertiesCache,
    pyramidIDCache,
    rating,
    rawSettings INTEGER,
    rootFile INTEGER,
    sidecarStatus
);
CREATE TABLE Adobe_libraryImageSavedSettings3 (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    dateCreated INTEGER,
    image INTEGER,
    isUserCheckpoint INTEGER NOT NULL DEFAULT 0,
    name,
    text,
    uuid
);
CREATE TABLE Adobe_namedIdentityPlate (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    description,
    identityPlate,
    identityPlateHash,
    moduleFont,
    moduleSelectedTextColor,
    moduleTextColor
);
CREATE TABLE Adobe_oldMetadataCache (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    image INTEGER,
    xmp
);
CREATE TABLE Adobe_variables (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    name,
    value
);
CREATE TABLE Adobe_variablesTable (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    name UNIQUE,
    value NOT NULL DEFAULT ''
);
CREATE TABLE AgLibraryContent (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    containingTag INTEGER,
    content,
    owningModule
);
CREATE TABLE AgLibraryImageSearchIndex (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    dirty INTEGER NOT NULL DEFAULT 0,
    image INTEGER NOT NULL DEFAULT 0,
    searchIndex
);
CREATE TABLE AgLibrarySpecialCollectionInfo (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    kind UNIQUE NOT NULL DEFAULT '',
    sortDirection,
    sortOrder
);
CREATE TABLE AgLibraryTag (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    dateCreated NOT NULL DEFAULT '',
    flattenedParams,
    genealogy NOT NULL DEFAULT '',
    imageCountFlag INTEGER NOT NULL DEFAULT 1,
    kindName NOT NULL DEFAULT '',
    lastApplied,
    name,
    parent INTEGER,
    sortDirection,
    sortOrder
);
CREATE TABLE AgLibraryTagImage (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    flagged,
    flattenedParams,
    image INTEGER NOT NULL DEFAULT 0,
    positionInCollection NOT NULL DEFAULT 'z',
    tag INTEGER NOT NULL DEFAULT 0,
    tagKind NOT NULL DEFAULT ''
);
CREATE TABLE AgLibraryTagSearchWord (
    id_local INTEGER PRIMARY KEY,
    id_global UNIQUE NOT NULL,
    implied INTEGER NOT NULL DEFAULT 0,
    tag INTEGER NOT NULL DEFAULT 0,
    word NOT NULL DEFAULT ''
);

sqlite> .quit
$

It would be prudent at this point to quote directly from Eric's post: