No, they are not. While it's true that our familiar system of so-called "Arabic numerals" (1, 2, 3, 4, 5 and so on) has become nearly ubiquitous around the world — particularly in math and science contexts — some cultures/languages still write numbers with different symbols. (And that's not even counting historical systems like Cyrillic, Hebrew and Roman numerals, which still exist but aren't really in everyday use.)

Interestingly, our "Arabic" numerals are not used in Arabic script. (It's probably more accurate to call our numerals "Western Arabic" numerals.) Arabic has its own numeral system; it's evolutionarily related to ours, yet looks notably different. (Further confusing things, the Arabs call their system "Hindi numerals" and call ours "Hindu numerals.") Here are the numbers 1 through 5 in "Eastern Arabic" numerals: ١ ٢ ٣ ٤ ٥. (If those don't show up in your browser, you can see a graphical representation here.) The Arab world, of course, is also familiar with "our" numerals. For example, you'll find them all over the Bank of Egypt website and on one side of Iraqi currency.

The reason we call our numerals "Arabic" is that Europeans learned them from Arab traders. However, the Arabs didn't invent them. They got them from Persian mathematicians, and the Persians picked them up in India, which is where they were really invented. (Actually, the Indians might have borrowed them from someone else.) Both Western Arabic and Eastern Arabic numerals are part of the larger "Hindu-Arabic" family of numeral systems, which has evolved into several different, but related, methods of writing numbers.

In India, for example, the Hindu-Arabic numeral system developed into Devanagari numerals, which are used in languages that employ the Devanagari alphabet (including Hindi, Marathi, Sanskrit and Nepali). The Devanagari numerals for 1 through 5 look like this: १ २ ३ ४ ५. (Again, click here to see them in a graphical format.) The similarities to our numerals should be fairly apparent.

Other languages that use variants of the Hindu-Arabic numeral system include Persian, Gujarati, Tamil, Tibetan, Thai, Lao and Khmer. (See Wikipedia: Hindu-Arabic numeral system for illustrations.)

In China, Western Arabic numerals were adapted in the 19th century for mathematical purposes, and they're commonly used for phone numbers, addresses, etc. But the Chinese language also has its own unique symbols for numbers, which can be seen on everything from signs to menus to mah-jongg tiles. The Chinese characters for the numbers 1 through 5 are: 一 二 三 四 五. (Click here if they don't display.) Chinese also has a separate set of "financial" numerals which are used in writing cheques to prevent forgeries. These are "fancier" characters (more strokes) that are harder to modify for illicit gain. (With the simple symbols, you can turn a 3 (三) into a 5 (五) with two lines.)

We should note that the Chinese writing system assigns single characters to words (syllables), not letters, so you could argue that these symbols are the Chinese equivalent of "one, two, three, four, five" — not "1, 2, 3, 4, 5" — and are thus different ways of writing words, not numerals.

Modern Japanese has numerals that are essentially the same as the Chinese characters, but also uses Arabic numerals widely. Korean has two different numeral systems; one is native Korean, the other comes from Chinese. But, again, the Koreans are very familiar with Arabic numerals. (Here's the home page of the Department of Mathematics at Korea University. You'll see a lot of recognizable symbols there, even if you can't read Korean.)

Many other languages that are spoken less widely, including Inuit, have their own numerals. Omniglot has a good selection of these on display in table form. If you look hard, you'll find correspondences in most of them.

See also: Wikipedia: List of numbers in various languages, Glossographia, Archimedes' Laboratory, A History of Mathematical Notations (on Google Books)