What sort of illegalness are you worried about? Is the text someone else's trademark? Are you worried that the shapes of the characters in the Arial font are protected? They're not, you can use them for anything you like. Arial itself is just a ripoff of Helvetica.
Arial is a trademarked font of Monotype Imaging. Technically you are supposed to get permission to use a font in such a manner, though with arial that may be an acceptable usage of the font under the licensing agreements already in place with MS and Apple. I know with other fonts you need to get permission if you want to use the font for commercial purposes (creating a business logo falls into this category). Legally the same is true of standard fonts, but as I said that permission may be written into the license agreements already in place with the people that provide the software that has arial as a standard font. That being said, even if it is not part of the agreement, I doubt anybody would come after you for it with arial (or any of the standard fonts). If you use a non-standard font, check the permissions set forth in the purchase agreement for the font. Most fonts give you permission to use them for commercial purposes when you buy the font, some require a higher purchase price for that type of use. Arial is not really a rippoff of Helvetica. It is a copy of the Grotesque font. Only the proportion of Helvetica were used so that Arial could be substituted for Helvetica without having to reformat for printing.
They may ask for permission, but at least in the USA (a widely watched jurisdiction for those of in the common-law backwaters) they have no demonstrated remedy if you don't get it. The software comprising a font distribution is protected under copyright, and the name may be protected as a trademark, but the shapes themselves are no more protected than a movie's plot line or other notional constructs. When you use a font to set type in Photoshop, and export it as an image, you are free to do whatever you want with it. I suppose there's a gray area somewhere in Illustrator/Freehand convert-to-paths territory, but so far - at least since 1976 - the courts have been on the side of font users.
That's not exactly true, fonts can be "protected" in the US but enforcement of that protection is much more difficult, so registering for that protection is not usually worth the cost for most font developers. Also, there have been a number of lawsuits revolving around fonts here in the US that have gone the way of the plaintiff. That being said, most of them involve large corporations (why sue somebody that has no money). And, as I said, standard fonts that are included may very well not be subject to these types of suits as the software maker paid the licensing fee in order to incorporate the font into the software. Arial is most definately a standard font.
Please cite any finding against an end user for use of character shapes from a commercial font. Good luck.
If you mean by commercial a font that is packaged with programs, then I don't think there is. As I stated, the license signed when the font gets packaged may very well cover this type of usage. If you mean by commercial font, a font that has commercial value, then there are plenty of cases. Emigre, for example, has gone after print shops for violating the terms of their license. Under their license, when you purchase the font you can use it on no more than six computers and only at the same physical location. You must purchase additional licenses if your shop has more than one location and a different license if you want to use the font on distributable materials. Any that I have found so far were settled out of court, which is still a win for the plaintiff. Not so oddly, all copyright claims for fonts center around computer fonts, which in some cases can be copyrighted as a software program. Traditional fonts cannot be copyrighted (though they may be trademarked). The small number of lawsuits filed does not reflect the legality of anything, it reflects the cost/benefit of suing. As I stated before, why sue somebody with no money. There is a reason that magazines, professional designers, and anybody in professional electronic-print media licenses fonts. Because they have to, not because they like spending money on things they can use for free.
From the beginning I have agreed that the software representation of a font is protected by copyright. My point is that the visual appearance of the characters is not. You can buy one copy and install it on one computer and then print 200 billion pages and make a trillion dollars in the process, and Emigre still doesn't get more than their $39, nor are they in any case entitled to. And that's what the original poster was (as far as I could understand) asking about.