Here are a few observations and questions about US phone number formatting on the N900. Not sure if it matters, but I'm on AT&T.

1a) In the Conversations app, if the phone number does not match exactly with an existing contact, it will show the phone number instead of contact name. For instance, if your contact called "Joe" is stored as 2125551234 and the number calling is 12125551234 (perhaps the carrier network adds the leading 1, but functionally there is no difference), Conversations will show the number and not "Joe". I recall my old E61 would handle these minor discrepancies quite elegantly and properly resolved the contact name. Would you consider this a deficiency/bug in the N900?
1b) In terms of making calls, the N900 does not seem to mind phone numbers being in various formats (e.g. 10 digit number, 1+10 digit number, parentheses, dashes).

2) I've been trying to figure out why some of my contact phone numbers are stored as just digits whereas others have a "pretty" format like (212)555-1234. When adding a contact number, the N900 does not let me input parentheses or dashes. Since most of my contacts have US numbers, it would be great if all the numbers showed up in "pretty" format. Although I did import most of my contacts from the E61, so it's possible the formatting was applied already (I'll try to verify this when I have access to the old phone), I don't recall having the numbers in pretty form previously.

3) I was under the impression that the N900 could handle typing in vanity numbers (e.g. 1-800-PROGRESSIVE), but that doesn't seem to be the case, at least when using the dialing pad in the Phone app.