From the US Delegate's address to the OSCE

The list of fraudulent and abusive practices reported by credible sources is extensive, and includes outright falsification of results in certain regions; physical intimidation of and violence against voters, election officials and observers; removal of official opposition commissioners, observers and journalists from polling stations; abuse of absentee certificates; coercion of students, public sector employees, and residents of governmental hospitals and prisons; multiple voting; ballot box stuffing; and inaccurate voter lists.


full text here

It sounds like there's a number of reasons to believe the election was fraudulent. Telling, I think, is Yuschenko's position (translated from pravda.com.ua by Tanya over at the periscope) which displays confidence that he would win a fair election:

Viktor Yushchenko reckons that the only way out of the political crisis can be the re-election of the President.
As Yushchenko stated at a demonstration in Kiev, the re-election has to be held on 12 December.
Also, there has to be a law to forbid the use of additional voting coupons, Central Election Committee (CEC) is to be dismissed and the new CEC is to be formed on the basis of equal representation from Yushchenko and Yanukovych side.
The third condition is an equal access of candidates to mass media, the fourth one is refusal to use the administrative resources.

Yanukovych's response?

Yanukovych offered to investigate the claims on the process of the election, which, according to Yushchenko, total 11 thousand. ‘It will be enough for us to investigate them till the end of our days!’, said Yushchenko.

...

At the same time, according to Yushchenko, he was under the impression that Yanukovych’s side wants to draw out the negotiations.
‘If Yanukovych aims to wear out the strength and draw out the time, we come to the active actions straight away’.

I think he's right. I believe the position of the authorities is to wait until the public's ire subsides - and hopefully assume the presidency as the fervor dies down. It's a good strategy, I think. Yushchenko would be best served to press the issue now - striking while the iron is hot, if you will. There's only so many nights a reasonable person can spend outside in a tent in the middle of November in the Ukraine, regardless of how much vodka you throw down. For now, at least, it would appear that the last "official" channel for redress is the Supreme Court hearing on Monday.