Katy Huberty’s three scenarios for Apple (AAPL) — bear, base and bull — is the lead item in Morgan Stanley’s “Key Surprises for 2010,” a 15-company forecast issued to clients Wednesday.

Huberty is leaning heavily toward the bull, offering three paths by which Apple hits somewhere between $325 and $435 per share within the next 12 months:

1. Apple broadens distribution for global market coverage, reaching 10% global handset market share — roughly one-third of the smartphone market — by the September 2012 fiscal year.
2. In addition to greater high-end smartphone penetration, Apple introduces products with more affordable hardware/service plans, increasing its handset market share further to 15% (roughly 50% of the smartphone market) in F2012.
3. Apple drives for handset market leadership by forgoing carrier subsidies and shifting the iPhone profit pool to content, services, accessories.

The Street, she believes, is only giving Apple a 5% chance of surpassing that $325-plus target. She puts the odds at one in four.

Her other two scenarios are not so rosy:

• Base Case Scenario: $230/share. iPhone success limited to high-end (F2012 market share 4%).
• Bear Case Scenario: $150/share. iPhone loses its luster, open ecosystems win (F2012 market share 2-3%).

Apple closed Wednesday at $195.03, up 0.86 points (0.44%) for the day.

See also:

• Morgan Stanley drinks the Apple Kool-Aid

[Follow Philip Elmer-DeWitt on Twitter @philiped]