Carrier options for US iPhone customers have dramatically multiplied over the past two years. The handset, which was once only available from AT&T, is now offered on nine wireless providers across the country.

Actually, 10 carriers if you count Straight Talk. You may or may not know this, but the nationwide prepaid provider is compatible with the GSM iPhone. You don’t even need to unlock your handset to switch to it…

How does it manage to pull this off? Well, Straight Talk is an MVNO (Mobile Virtual Network Operator) of sorts. It piggybacks on Verizon and Sprint networks for CDMA coverage, and AT&T and T-Mobile for GSM.

When you sign up for the service, you pick your carrier and your SIM card — it comes in both standard and micro. So for instance, you could order an AT&T-branded micro SIM, and your iPhone would never know the difference.

Why would anyone want to switch to Straight Talk? Price. The operator easily undercuts the larger carriers by $50 or more with its $45 all-you-can-eat plan. That’s unlimited voice minutes, text messaging and HSPA+ data. Wow.

While this all sounds great, we haven’t actually tried this for ourselves (yet). So we don’t know what Straight Talk’s coverage or service is like, and we aren’t sure if all of the iPhone’s features — Visual Voicemail, MMS, etc. — continue to work.

Still, for $45 a month and no hassle of unlocking your handset, Straight Talk appears to be a viable carrier option for those who are looking for a way to dramatically cut down their monthly expenses.

Are you using your iPhone on Straight Talk? We’d love to hear about it below.

[GigaOM]