Have you ever wanted to create page numbering (such as "Pages 4-5") on one page of the spread? Me, neither. But there must be some trendy new design style that puts both spread numbers on a single page. That's the only way I can account for the number of times this issue has come up in the forums.

The good news is that it's relatively easy to do in InDesign. The bad news is that a bug prevents it from working smoothly. The good news is that there's a fairly simple workaround. If you're keeping score at home, that's Good-2, Bad-1.
Here's how to create this quirky page numbering technique.
Step 1. In the Pages panel, open the master page where you want to add the page numbering, and create a text frame. In my case, I'm putting the numbering on the right master. Type any text that'll go along with the page numbering, and format the text.
For example, you may want to use a paragraph style that right-aligns the text and applies a font style used for headers.
Step 2. Choose Type > Insert Special Characters > Markers > Previous Page Number to insert the page number from the left-hand page. Insert a hyphen (or en dash), and then choose Type > Insert Special Characters > Markers > Current Page Number.

Of course, if you're adding the page numbering to the left master, you'll want to insert the current page number first and then insert the Next Page Number character after the hyphen.
In a perfect world, you'd be done. But there's a catch -- and we haven't even gotten to the real bug yet. You have to thread the text frame on the right-hand page to a text frame on the left-hand page. You need to do this because the Previous Page Number and Next Page Number work on a story level, not on a document level.
Step 3. Create a text frame on the left master page, and thread it to the text frame on the right master page (click the out port, and then click the other text frame). Then place the insertion point at the beginning of the text that belongs in the next frame and choose Type > Insert Break Character > Frame Break.
It should work, right? Wrong. If you go to a document page to which the master is applied, you'll see the Previous Page Number is the same as the Current Page Number.

Here's where the bug enters the picture. For some reason, InDesign stumbles on the Previous Page Number if it's in a text frame that's based on a master page. Until this bug gets fixed, you need to override the master page item on every page where it appears. Ouch. The program team is aware of this bug, and I'm hoping it gets fixed by the end of summer.
Step 4. On each document page to which the master is applied, hold down Ctrl/Command+Shift and click the text frame with the page numbering.

Now that I've written out the steps, I think a more accurate score is Good-2, Bad-2. I'll revise this when the bug gets fixed.

In CS3 you can do this by making a text frame that will take the previous number in the left page and inserting the page marker, then stretching the frame to the other page. In CS4 however, it doesn't work. Adobe says the bug is fixed as of july 06, '09.
I don't think it's fixed yet in CS4. I think you still need to override the master page item to update the numbering.
I created my document in CS3 where it was working seamlesly as in all later versions. After conversion into CS4 I noticed this „little“ fault very late. It is September 15 and it still doesnt work. „Till the end of summer“ you ment September 23?
Pavel.
there are any news about that bug?
Nope. It's still broken in the newest update.
I wrote a script to solve this problem. I'm not sure how to format long scripts in MovableType, so I found a similar question on the Adobe InDesign Forums and posted the answer and script there.
I hope it's useful for someone!